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SubjectRE: [PATCH V8 3/5] i2c: tegra: Add DMA Support
Date
> > > > > +	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
Pasting again here with clear comment to explain on why I have those corresponding dma_sync

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 */
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 */
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 */
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);

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



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