Re: [PATCH v2 5/8] dma: dw: Avoid partial transfers

[Miquel Raynal <miquel.raynal@bootlin.com>]

Hi Andy, Phil,

andriy.shevchenko@linux.intel.com wrote on Wed, 23 Feb 2022 15:35:58
+0200:

> On Tue, Feb 22, 2022 at 11:34:34AM +0100, Miquel Raynal wrote:
> > From: Phil Edworthy <phil.edworthy@renesas.com>
> > 
> > Pausing a partial transfer only causes data to be written to memory that
> > is a multiple of the memory width setting.
> > 
> > However, when a DMA client driver finishes DMA early, e.g. due to UART
> > char timeout interrupt, all data read from the device must be written to
> > memory.
> > 
> > Therefore, allow the slave to limit the memory width to ensure all data
> > read from the device is written to memory when DMA is paused.  
> 
> (I have only 2.17d and 2.21a datasheets, so below based on the latter)
> 
> It seems you are referring to the chapter 7.7 "Disabling a Channel Prior
> to Transfer Completion" of the data sheet where it stays that it does not
> guarantee to have last burst to be completed in case of
> SRC_TR_WIDTH < DST_TR_WIDTH and the CH_SUSP bit is high, when the FIFO_EMPTY
> is asserted.
> 
> Okay, in iDMA 32-bit we have a specific bit (seems like a fix) that drains
> FIFO, but still it doesn't drain the FIFO fully (in case of misalignment)
> and the last piece of data (which is less than TR width) is lost when channel
> gets disabled.
> 
> Now, if we look at the implementation of the serial8250_rx_dma_flush() we
> may see that it does
>  1. Pause channel without draining FIFO
>  2. Moves data to TTY buffer
>  3. Terminates channel.
> 
> During termination it does pause channel again (with draining enabled),
> followed by disabling channel and resuming it again.
> 
> According to the 7.7 the resuming channel allows to finish the transfer
> normally.
> 
> It seems the logic in the ->terminate_all() is broken and we actually need
> to resume channel first (possibly conditionally, if it was suspended), then
> pause it and disable and resume again.
> 
> The problem with ->terminate_all() is that it has no knowledge if it has
> been called on paused channel (that's why it has to pause channel itself).
> The pause on termination is required due to some issues in early steppings
> of iDMA 32-bit hardware implementations.
> 
> If my theory is correct, the above change should fix the issues you see.

I don't have access to these datasheets so I will believe your words
and try to apply Andy's solution. I ended up with the following fix,
hopefully I got it right:

diff --git a/drivers/dma/dw/core.c b/drivers/dma/dw/core.c
index 48cdefe997f1..59822664d8ec 100644
--- a/drivers/dma/dw/core.c
+++ b/drivers/dma/dw/core.c
@@ -865,6 +865,10 @@ static int dwc_terminate_all(struct dma_chan *chan)
 
        clear_bit(DW_DMA_IS_SOFT_LLP, &dwc->flags);
 
+       /* Ensure the last byte(s) are drained before disabling the channel */
+       if (test_bit(DW_DMA_IS_PAUSED, &dwc->flags))
+               dwc_chan_resume(dwc, true);
+
        dwc_chan_pause(dwc, true);
 
        dwc_chan_disable(dw, dwc);

Phil, I know it's been 3 years since you investigated this issue, but
do you still have access to the script reproducing the issue? Even
better, do you still have the hardware to test?

Thanks,
Miquèl