| Message ID | 952745481.440383.1494666448727@email.1und1.de (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Sat, May 13, 2017 at 11:07:28AM +0200, Stefan Wahren wrote: > In the meantime this issue has been fixed by Phil [1]. Right - definitely a driver bug. Mapping more memory for DMA than is actually going to be DMA'd to and expecting data to be preserved is really horrid. > Unfortunately i found another issue. If i enable CONFIG_HIGHMEM in > the kernel config, the data during functional test gets corrupted. > Phil said it's caused by the usage of get_user_pages() [2]. Without knowing who "Phil" is in that thread, but... HIGHMEM is a problem because you can't use get_user_pages on pages in HIGHMEM. is an interesting statement, and without any reasoning or evidence. I also believe it to be incorrect. get_user_pages() returns an array of struct page pointers for the user memory, calling flush_dcache_page() and flush_anon_page() on them to ensure that any kernel mapping is coherent with what is in userspace. As far as returning the array of page pointers, get_user_pages() doesn't care whether they're lowmem or highmem. flush_dcache_page() doesn't care either - if it wants to flush the page and the page is a highmem page, it will temporarily map it before flushing it. flush_anon_page() is a no-op for all non-aliasing caches. get_user_pages() works fine for whatever memory on other platforms and drivers such as etnaviv, so I think this comes down to the vchip driver doing things in ways that the kernel interfaces its using don't expect - exactly like the "lets pass full pages to the DMA API" broken-ness. I would like to hear the justification for that statement, but without any justification, I assert that the statement is false.
On 13/05/2017 10:30, Russell King - ARM Linux wrote: > On Sat, May 13, 2017 at 11:07:28AM +0200, Stefan Wahren wrote: >> In the meantime this issue has been fixed by Phil [1]. > > Right - definitely a driver bug. Mapping more memory for DMA than is > actually going to be DMA'd to and expecting data to be preserved is > really horrid. That feature was added during the upstreaming process, and as Stefan says there is an outstanding patch for it. >> Unfortunately i found another issue. If i enable CONFIG_HIGHMEM in >> the kernel config, the data during functional test gets corrupted. >> Phil said it's caused by the usage of get_user_pages() [2]. > > Without knowing who "Phil" is in that thread, but... > > HIGHMEM is a problem because you can't use get_user_pages on pages in > HIGHMEM. > > is an interesting statement, and without any reasoning or evidence. > > I also believe it to be incorrect. get_user_pages() returns an array > of struct page pointers for the user memory, calling flush_dcache_page() > and flush_anon_page() on them to ensure that any kernel mapping is > coherent with what is in userspace. > > As far as returning the array of page pointers, get_user_pages() doesn't > care whether they're lowmem or highmem. > > flush_dcache_page() doesn't care either - if it wants to flush the page > and the page is a highmem page, it will temporarily map it before > flushing it. > > flush_anon_page() is a no-op for all non-aliasing caches. > > get_user_pages() works fine for whatever memory on other platforms and > drivers such as etnaviv, so I think this comes down to the vchip driver > doing things in ways that the kernel interfaces its using don't expect - > exactly like the "lets pass full pages to the DMA API" broken-ness. See previous comment. > I would like to hear the justification for that statement, but without > any justification, I assert that the statement is false. I am the Phil in question, and the off-the-cuff comment was the result of a hazy memory of issues encountered with VCHIQ bulk transfers as a Broadcom employee (which would have been on a 2.6 kernel). I suspect there may have been some use of kernel virtual addresses as an intermediate representation, but I no longer have access to that code. If get_user_pages is HIGHMEM-safe (and I can see why it would be), then the cause of the corruption Stefan saw is probably the special handling of unaligned reads, specifically: memcpy((char *)page_address(pages[0]) + pagelist->offset, fragments, head_bytes); Phil
Am 15.05.2017 um 16:29 schrieb Phil Elwell: > On 13/05/2017 10:30, Russell King - ARM Linux wrote: >> On Sat, May 13, 2017 at 11:07:28AM +0200, Stefan Wahren wrote: >>> In the meantime this issue has been fixed by Phil [1]. >> Right - definitely a driver bug. Mapping more memory for DMA than is >> actually going to be DMA'd to and expecting data to be preserved is >> really horrid. > That feature was added during the upstreaming process, and as Stefan says > there is an outstanding patch for it. > >>> Unfortunately i found another issue. If i enable CONFIG_HIGHMEM in >>> the kernel config, the data during functional test gets corrupted. >>> Phil said it's caused by the usage of get_user_pages() [2]. >> Without knowing who "Phil" is in that thread, but... >> >> HIGHMEM is a problem because you can't use get_user_pages on pages in >> HIGHMEM. >> >> is an interesting statement, and without any reasoning or evidence. >> >> I also believe it to be incorrect. get_user_pages() returns an array >> of struct page pointers for the user memory, calling flush_dcache_page() >> and flush_anon_page() on them to ensure that any kernel mapping is >> coherent with what is in userspace. >> >> As far as returning the array of page pointers, get_user_pages() doesn't >> care whether they're lowmem or highmem. >> >> flush_dcache_page() doesn't care either - if it wants to flush the page >> and the page is a highmem page, it will temporarily map it before >> flushing it. >> >> flush_anon_page() is a no-op for all non-aliasing caches. >> >> get_user_pages() works fine for whatever memory on other platforms and >> drivers such as etnaviv, so I think this comes down to the vchip driver >> doing things in ways that the kernel interfaces its using don't expect - >> exactly like the "lets pass full pages to the DMA API" broken-ness. > See previous comment. > >> I would like to hear the justification for that statement, but without >> any justification, I assert that the statement is false. > I am the Phil in question, and the off-the-cuff comment was the result of > a hazy memory of issues encountered with VCHIQ bulk transfers as a Broadcom > employee (which would have been on a 2.6 kernel). I suspect there may have > been some use of kernel virtual addresses as an intermediate representation, > but I no longer have access to that code. > > If get_user_pages is HIGHMEM-safe (and I can see why it would be), then the > cause of the corruption Stefan saw is probably the special handling of > unaligned reads, specifically: > > memcpy((char *)page_address(pages[0]) + > pagelist->offset, > fragments, > head_bytes); Btw shouldn't we use copy_from_user() at this place?
On 15/05/2017 15:54, Stefan Wahren wrote: > Am 15.05.2017 um 16:29 schrieb Phil Elwell: >> On 13/05/2017 10:30, Russell King - ARM Linux wrote: >>> On Sat, May 13, 2017 at 11:07:28AM +0200, Stefan Wahren wrote: >>>> In the meantime this issue has been fixed by Phil [1]. >>> Right - definitely a driver bug. Mapping more memory for DMA than is >>> actually going to be DMA'd to and expecting data to be preserved is >>> really horrid. >> That feature was added during the upstreaming process, and as Stefan says >> there is an outstanding patch for it. >> >>>> Unfortunately i found another issue. If i enable CONFIG_HIGHMEM in >>>> the kernel config, the data during functional test gets corrupted. >>>> Phil said it's caused by the usage of get_user_pages() [2]. >>> Without knowing who "Phil" is in that thread, but... >>> >>> HIGHMEM is a problem because you can't use get_user_pages on pages in >>> HIGHMEM. >>> >>> is an interesting statement, and without any reasoning or evidence. >>> >>> I also believe it to be incorrect. get_user_pages() returns an array >>> of struct page pointers for the user memory, calling flush_dcache_page() >>> and flush_anon_page() on them to ensure that any kernel mapping is >>> coherent with what is in userspace. >>> >>> As far as returning the array of page pointers, get_user_pages() doesn't >>> care whether they're lowmem or highmem. >>> >>> flush_dcache_page() doesn't care either - if it wants to flush the page >>> and the page is a highmem page, it will temporarily map it before >>> flushing it. >>> >>> flush_anon_page() is a no-op for all non-aliasing caches. >>> >>> get_user_pages() works fine for whatever memory on other platforms and >>> drivers such as etnaviv, so I think this comes down to the vchip driver >>> doing things in ways that the kernel interfaces its using don't expect - >>> exactly like the "lets pass full pages to the DMA API" broken-ness. >> See previous comment. >> >>> I would like to hear the justification for that statement, but without >>> any justification, I assert that the statement is false. >> I am the Phil in question, and the off-the-cuff comment was the result of >> a hazy memory of issues encountered with VCHIQ bulk transfers as a Broadcom >> employee (which would have been on a 2.6 kernel). I suspect there may have >> been some use of kernel virtual addresses as an intermediate representation, >> but I no longer have access to that code. >> >> If get_user_pages is HIGHMEM-safe (and I can see why it would be), then the >> cause of the corruption Stefan saw is probably the special handling of >> unaligned reads, specifically: >> >> memcpy((char *)page_address(pages[0]) + >> pagelist->offset, >> fragments, >> head_bytes); > > > Btw shouldn't we use copy_from_user() at this place? I'm sure you mean copy_to_user(), and the answer is "it's complicated". Depending on the relative timing, this code can also be called from a kernel thread, so I doubt that would work. Phil
diff --git a/drivers/staging/vc04_services/Kconfig b/drivers/staging/vc04_servic index 9e27636..6572903 100644 --- a/drivers/staging/vc04_services/Kconfig +++ b/drivers/staging/vc04_services/Kconfig @@ -2,7 +2,7 @@ menuconfig BCM_VIDEOCORE tristate "Broadcom VideoCore support" depends on HAS_DMA depends on OF - depends on RASPBERRYPI_FIRMWARE || (COMPILE_TEST && !RASPBERRYPI_FIRMWAR + depends on (RASPBERRYPI_FIRMWARE && !HIGHMEM) || (COMPILE_TEST && !RASPB default y help Support for Broadcom VideoCore services including