linux-kernel - Re: [PATCH 2/2] spi: spi-geni-qcom: Do not do DMA map/unmap inside driver, use framework instead

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Date:   Wed, 17 May 2023 07:23:23 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Vijaya Krishna Nivarthi <quic_vnivarth@...cinc.com>
Cc:     agross@...nel.org, andersson@...nel.org, konrad.dybcio@...aro.org,
        broonie@...nel.org, linux-arm-msm@...r.kernel.org,
        linux-spi@...r.kernel.org, linux-kernel@...r.kernel.org,
        quic_msavaliy@...cinc.com, mka@...omium.org, swboyd@...omium.org,
        quic_vtanuku@...cinc.com, quic_ptalari@...cinc.com
Subject: Re: [PATCH 2/2] spi: spi-geni-qcom: Do not do DMA map/unmap inside
 driver, use framework instead

Hi,

On Wed, May 17, 2023 at 5:17 AM Vijaya Krishna Nivarthi
<quic_vnivarth@...cinc.com> wrote:
>
> > One last question: should you call:
> >
> > dma_set_max_seg_size(dev, INT_MAX)
> >
> > ...in your probe function? I don't think you have any limitations of
> > maximum segment size, right? Right now if you don't set anything it
> > looks as if it considers your max to be 64K. That would cause the SPI
> > framework to break things up into multiple chunks which would make you
> > fall back to FIFO mode, right?
>
>
> Actually we would need to call:
>
> dma_set_max_seg_size(dev->parent, INT_MAX)
>
> Please note that in probe()
>
> spi->dma_map_dev = dev->parent;
>
> and in __spi_map_msg()
>
> tx_dev = ctlr->dma_map_dev;
>
> ret = spi_map_buf(ctlr, tx_dev, &xfer->tx_sg,...
>
>
> Since the dev->parent is QUP containing other SEs and its max_seg_size
> seems to be getting set from elsewhere than code (perhaps kernel
> scripts) it seemed safer not to modify that.
>
> So I made below change and uploaded v2...
>
> spi->max_dma_len = 0xffff0; /* 24 bits for tx/rx dma length */
>
> which actually doesnt help much because spi_map_buf() picks the lower of
> these 2 but added it anyway
>
> desc_len = min_t(size_t, max_seg_size, ctlr->max_dma_len);

OK, fair enough.


> Any case as next step we will look into scatter list support to DMA; and
> practically we may not have transfers over 64KB, so its ok for now?

I think it's OK for now, especially since you do properly fall back to
FIFO in cases that aren't handled. Thanks!

-Doug