| Message ID | 20170920185956.13874-4-wsa+renesas@sang-engineering.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Wed, 20 Sep 2017 16:56:48 -0300 Mauro Carvalho Chehab <mchehab@s-opensource.com> wrote: > Em Wed, 20 Sep 2017 20:59:53 +0200 > Wolfram Sang <wsa+renesas@sang-engineering.com> escreveu: > > > Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com> > > Documentation looks OK on my eyes. So: > > Reviewed-by: Mauro Carvalho Chehab <mchehab@s-opensource.com> Really minor suggestion inline. I don't really care either way as what you had is perfectly comprehensible. Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com> > > > --- > > Documentation/i2c/DMA-considerations | 58 ++++++++++++++++++++++++++++++++++++ > > 1 file changed, 58 insertions(+) > > create mode 100644 Documentation/i2c/DMA-considerations > > > > diff --git a/Documentation/i2c/DMA-considerations b/Documentation/i2c/DMA-considerations > > new file mode 100644 > > index 00000000000000..5a63355c6a9b6f > > --- /dev/null > > +++ b/Documentation/i2c/DMA-considerations > > @@ -0,0 +1,58 @@ > > +================= > > +Linux I2C and DMA > > +================= > > + > > +Given that I2C is a low-speed bus where largely small messages are transferred, Slightly nicer as: Given that i2c is a low-speed bus, over which the majority of messages transferred are small, > > +it is not considered a prime user of DMA access. At this time of writing, only > > +10% of I2C bus master drivers have DMA support implemented. And the vast > > +majority of transactions are so small that setting up DMA for it will likely > > +add more overhead than a plain PIO transfer. > > + > > +Therefore, it is *not* mandatory that the buffer of an I2C message is DMA safe. > > +It does not seem reasonable to apply additional burdens when the feature is so > > +rarely used. However, it is recommended to use a DMA-safe buffer if your > > +message size is likely applicable for DMA. Most drivers have this threshold > > +around 8 bytes (as of today, this is mostly an educated guess, however). For > > +any message of 16 byte or larger, it is probably a really good idea. Please > > +note that other subsystems you use might add requirements. E.g., if your > > +I2C bus master driver is using USB as a bridge, then you need to have DMA > > +safe buffers always, because USB requires it. > > + > > +For clients, if you use a DMA safe buffer in i2c_msg, set the I2C_M_DMA_SAFE > > +flag with it. Then, the I2C core and drivers know they can safely operate DMA > > +on it. Note that using this flag is optional. I2C host drivers which are not > > +updated to use this flag will work like before. And like before, they risk > > +using an unsafe DMA buffer. To improve this situation, using I2C_M_DMA_SAFE in > > +more and more clients and host drivers is the planned way forward. Note also > > +that setting this flag makes only sense in kernel space. User space data is > > +copied into kernel space anyhow. The I2C core makes sure the destination > > +buffers in kernel space are always DMA capable. > > + > > +FIXME: Need to implement i2c_master_{send|receive}_dma and proper buffers for i2c_smbus_xfer_emulated. > > + > > +Drivers wishing to implement safe DMA can use helper functions from the I2C > > +core. One gives you a DMA-safe buffer for a given i2c_msg as long as a certain > > +threshold is met:: > > + > > + dma_buf = i2c_get_dma_safe_msg_buf(msg, threshold_in_byte); > > + > > +If a buffer is returned, it is either msg->buf for the I2C_M_DMA_SAFE case or a > > +bounce buffer. But you don't need to care about that detail, just use the > > +returned buffer. If NULL is returned, the threshold was not met or a bounce > > +buffer could not be allocated. Fall back to PIO in that case. > > + > > +In any case, a buffer obtained from above needs to be released. It ensures data > > +is copied back to the message and a potentially used bounce buffer is freed:: > > + > > + i2c_release_dma_safe_msg_buf(msg, dma_buf); > > + > > +The bounce buffer handling from the core is generic and simple. It will always > > +allocate a new bounce buffer. If you want a more sophisticated handling (e.g. > > +reusing pre-allocated buffers), you are free to implement your own. > > + > > +Please also check the in-kernel documentation for details. The i2c-sh_mobile > > +driver can be used as a reference example how to use the above helpers. > > + > > +Final note: If you plan to use DMA with I2C (or with anything else, actually) > > +make sure you have CONFIG_DMA_API_DEBUG enabled during development. It can help > > +you find various issues which can be complex to debug otherwise. > > > > Thanks, > Mauro > -- > To unsubscribe from this list: send the line "unsubscribe linux-iio" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/Documentation/i2c/DMA-considerations b/Documentation/i2c/DMA-considerations new file mode 100644 index 00000000000000..5a63355c6a9b6f --- /dev/null +++ b/Documentation/i2c/DMA-considerations @@ -0,0 +1,58 @@ +================= +Linux I2C and DMA +================= + +Given that I2C is a low-speed bus where largely small messages are transferred, +it is not considered a prime user of DMA access. At this time of writing, only +10% of I2C bus master drivers have DMA support implemented. And the vast +majority of transactions are so small that setting up DMA for it will likely +add more overhead than a plain PIO transfer. + +Therefore, it is *not* mandatory that the buffer of an I2C message is DMA safe. +It does not seem reasonable to apply additional burdens when the feature is so +rarely used. However, it is recommended to use a DMA-safe buffer if your +message size is likely applicable for DMA. Most drivers have this threshold +around 8 bytes (as of today, this is mostly an educated guess, however). For +any message of 16 byte or larger, it is probably a really good idea. Please +note that other subsystems you use might add requirements. E.g., if your +I2C bus master driver is using USB as a bridge, then you need to have DMA +safe buffers always, because USB requires it. + +For clients, if you use a DMA safe buffer in i2c_msg, set the I2C_M_DMA_SAFE +flag with it. Then, the I2C core and drivers know they can safely operate DMA +on it. Note that using this flag is optional. I2C host drivers which are not +updated to use this flag will work like before. And like before, they risk +using an unsafe DMA buffer. To improve this situation, using I2C_M_DMA_SAFE in +more and more clients and host drivers is the planned way forward. Note also +that setting this flag makes only sense in kernel space. User space data is +copied into kernel space anyhow. The I2C core makes sure the destination +buffers in kernel space are always DMA capable. + +FIXME: Need to implement i2c_master_{send|receive}_dma and proper buffers for i2c_smbus_xfer_emulated. + +Drivers wishing to implement safe DMA can use helper functions from the I2C +core. One gives you a DMA-safe buffer for a given i2c_msg as long as a certain +threshold is met:: + + dma_buf = i2c_get_dma_safe_msg_buf(msg, threshold_in_byte); + +If a buffer is returned, it is either msg->buf for the I2C_M_DMA_SAFE case or a +bounce buffer. But you don't need to care about that detail, just use the +returned buffer. If NULL is returned, the threshold was not met or a bounce +buffer could not be allocated. Fall back to PIO in that case. + +In any case, a buffer obtained from above needs to be released. It ensures data +is copied back to the message and a potentially used bounce buffer is freed:: + + i2c_release_dma_safe_msg_buf(msg, dma_buf); + +The bounce buffer handling from the core is generic and simple. It will always +allocate a new bounce buffer. If you want a more sophisticated handling (e.g. +reusing pre-allocated buffers), you are free to implement your own. + +Please also check the in-kernel documentation for details. The i2c-sh_mobile +driver can be used as a reference example how to use the above helpers. + +Final note: If you plan to use DMA with I2C (or with anything else, actually) +make sure you have CONFIG_DMA_API_DEBUG enabled during development. It can help +you find various issues which can be complex to debug otherwise.
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com> --- Documentation/i2c/DMA-considerations | 58 ++++++++++++++++++++++++++++++++++++ 1 file changed, 58 insertions(+) create mode 100644 Documentation/i2c/DMA-considerations