Re: [PATCH V8 3/5] i2c: tegra: Add DMA Support

Dmitry Osipenko
2019-02-01T15:02:24Z

01.02.2019 7:19, Sowjanya Komatineni пишет:
>>>>>> + if (dma) {
>>>>>> + if (i2c_dev->msg_read) {
>>>>>> + chan = i2c_dev->rx_dma_chan;
>>>>>> + tegra_i2c_config_fifo_trig(i2c_dev,
>>>>>> xfer_size,
>>>>>> +
>>>>>> DATA_DMA_DIR_RX);
>>>>>> +
>>>>>> dma_sync_single_for_device(i2c_dev->dev, + i2c_dev->dma_phys,
>>>>>> + xfer_size,
>>>>>> +
>>>>>> DMA_FROM_DEVICE);
>>>>>
>>>>> Do we really need this? We're not actually passing the device any
>>>>> data, so no caches to flush here. I we're cautious about flushing
>>>>> caches when we do write to the buffer (and I think we do that
>>>>> properly already), then there should be no need to do it here
>>>>> again.
>>>>
>>>> IIUC, DMA API has a concept of buffer handing which tells to use
>>> dma_sync_single_for_device() before issuing hardware job that touches
>>> the buffer and to use dma_sync_single_for_cpu() after hardware done
>>> the execution. In fact the CPU caches are getting flushed or
>>> invalidated as appropriate in a result.
>>>>
>>>> dma_sync_single_for_device(DMA_FROM_DEVICE) invalidates buffer in
>>>> the CPU cache, probably to avoid CPU evicting data from cache to
>>>> DRAM while hardware writes to the buffer. Hence this hunk is
>>>> correct.
>>>>>> + err = tegra_i2c_dma_submit(i2c_dev,
>>>>>> xfer_size);
>>>>>> + if (err < 0) {
>>>>>> + dev_err(i2c_dev->dev,
>>>>>> + "starting RX DMA
>>>>>> failed, err %d\n",
>>>>>> + err);
>>>>>> + goto unlock;
>>>>>> + }
>>>>>> + } else {
>>>>>> + chan = i2c_dev->tx_dma_chan;
>>>>>> + tegra_i2c_config_fifo_trig(i2c_dev,
>>>>>> xfer_size,
>>>>>> +
>>>>>> DATA_DMA_DIR_TX);
>>>>>> + dma_sync_single_for_cpu(i2c_dev->dev,
>>>>>> +
>>>>>> i2c_dev->dma_phys,
>>>>>> + xfer_size,
>>>>>> +
>>>>>> DMA_TO_DEVICE);
>>>>>
>>>>> This, on the other hand seems correct because we need to
>>>>> invalidate the caches for this buffer to make sure the data that
>>>>> we put there doesn't get overwritten.
>>>>
>>>> As I stated before in a comment to v6, this particular case of
>>>> dma_sync_single_for_cpu() usage is incorrect because CPU should take
>>>> ownership of the buffer after completion of hardwate job. But in
>>>> fact dma_sync_single_for_cpu(DMA_TO_DEVICE) is a NO-OP because CPU
>>>> doesn't need to flush or invalidate anything to take ownership of
>>>> the buffer if hardware did a read-only access.
>>>>>
>>>>>> + if (!i2c_dev->msg_read) {
>>>>>> + if (dma) {
>>>>>> + memcpy(buffer, msg->buf, msg->len);
>>>>>> +
>>>>>> dma_sync_single_for_device(i2c_dev->dev, + i2c_dev->dma_phys,
>>>>>> + xfer_size,
>>>>>> +
>>>>>> DMA_TO_DEVICE);
>>>>>
>>>>> Again, here we properly flush the caches to make sure the data
>>>>> that we've written to the DMA buffer is visible to the DMA engine.
>>>>>
>>>>
>>>> +1 this is correct
>>>>
>>>>
>>>>
>>>>>> +
>>>>>> + if (i2c_dev->msg_read) {
>>>>>> + if (likely(i2c_dev->msg_err ==
>>>>>> I2C_ERR_NONE)) {
>>>>>> +
>>>>>> dma_sync_single_for_cpu(i2c_dev->dev,
>>>>>> +
>>>>>> i2c_dev->dma_phys,
>>>>>> +
>>>>>> xfer_size, +
>>>>>> DMA_FROM_DEVICE);
>>>>>
>>>>> Here we invalidate the caches to make sure we don't get stale data
>>>>> that may be in the caches for data that we're copying out of the
>>>>> DMA buffer. I think that's about all the cache maintenance that we
>>>>> real need.
>>>>
>>>> Correct.
>>>>
>>>> And technically here should be
>>>> dma_sync_single_for_cpu(DMA_TO_DEVICE) for the TX. But again, it's a
>>>> NO-OP.
>>>
>>> Is my below understanding correct? Can you please confirm?
>>>
>>> During Transmit to device:
>>> - Before writing msg data into dma buf by CPU, giving DMA ownership to
>>> CPU dma_sync_single_for_cpu with dir DMA_TO_DEVICE
>>>
>>
>> I tried to take a look at it again and now thinking that your variant is more correct. Still it's a bit difficult to judge because this case is no-op.
>>
>>> - After writing to dma buf by CPU, giving back the ownership to device
>>> to access buffer to send during DMA transmit
>>> dma_sync_single_for_device with dir DMA_TO_DEVICE
>>
>> Correct.
>>
>>> During Receiving from Device:
>>> - before submitting RX DMA to give buffer access to DMAengine
>>> dma_sync_single_for_Device(DMA_FROM_DEVICE)
>>
>> Correct.
>>
>>> - after DMA RX completion, giving dma ownership to CPU for reading
>>> dmabuf data written by DMA from device dma_sync_single_for_cpu with
>>> dir DMA_FROM_DEVICE
>>>
>>
>> Correct.
>
> Then what I have is exact as mentioned above. So no changes needed related to dma_sync

Yes

> Pasting again here with clear comment to explain on why I have those corresponding dma_sync

If you're going to add these comments to the actual patch, then please add brief explanation of what actually syncing is doing.

>
> if (i2c_dev->msg_read) {
> tegra_i2c_config_fifo_trig(i2c_dev, xfer_size,
> DATA_DMA_DIR_RX);
> /* For Reads: giving dma buf ownership to device before submitting RX DMA */

This invalidates buffer in CPU caches to avoid buffer-data eviction from the caches to DRAM while hardware writes to the buffer.

> dma_sync_single_for_device(i2c_dev->dev,
> i2c_dev->dma_phys,
> xfer_size,
> DMA_FROM_DEVICE);
> err = tegra_i2c_dma_submit(i2c_dev, xfer_size);
> if (err < 0) {
> dev_err(i2c_dev->dev,
> "starting RX DMA failed, err %d\n",
> err);
> goto unlock;
> }
> } else {
> tegra_i2c_config_fifo_trig(i2c_dev, xfer_size,
> DATA_DMA_DIR_TX);
> /* For writes: giving dma buf ownership to CPU to copy transmit data to DMA Buf */

This is a NO-OP because there is no need to flush nor to invalidate CPU caches.

> dma_sync_single_for_cpu(i2c_dev->dev,
> i2c_dev->dma_phys,
> xfer_size,
> DMA_TO_DEVICE);
> buffer = i2c_dev->dma_buf;
> }
>
>
>
> if (!msg->flags & I2C_M_RD) {
> if (dma) {
> memcpy(buffer, msg->buf, msg->len);
> /* For writes: giving ownership to device after done with copying data to DMA Buf */

This flushes out buffer from CPU caches to DRAM.

> dma_sync_single_for_device(i2c_dev->dev,
> i2c_dev->dma_phys,
> xfer_size,
> DMA_TO_DEVICE);
> err = tegra_i2c_dma_submit(i2c_dev, xfer_size);
> if (err < 0) {
> dev_err(i2c_dev->dev,
> "starting TX DMA failed, err %d\n",
> err);
> goto unlock;
> }
> } else {
> tegra_i2c_fill_tx_fifo(i2c_dev);
> }
>
> if (i2c_dev->msg_read) {
> if (likely(i2c_dev->msg_err == I2C_ERR_NONE)) {
> /* For Reads: giving ownership to CPU after RX DMA completion to access read data */
> dma_sync_single_for_cpu(i2c_dev->dev,
> i2c_dev->dma_phys,
> xfer_size,
> DMA_FROM_DEVICE);

This invalidates buffer in CPU caches again, even though it shall be kept invalidated after dma_sync_single_for_device(DMA_FROM_DEVICE). So it's likely to be a mostly NO-OP in hardware.

>
> memcpy(i2c_dev->msg_buf, i2c_dev->dma_buf,
> msg->len);
> }
> }
>
>
>

Technically we could remove the NO-OP's, but I personally would prefer to keep them for consistency with the DMA API.