linux-kernel - Re: [PATCH 1/3] dt-bindings: mmc: mmci: add a property to disable DMA LLI

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Date:   Thu, 17 Mar 2022 11:28:18 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Yann Gautier <yann.gautier@...s.st.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mmc@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Christophe Kerello <christophe.kerello@...s.st.com>,
        Ludovic Barre <ludovic.barre@...s.st.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Russell King <linux@...linux.org.uk>,
        Marek Vasut <marex@...x.de>, kernel@...electronics.com,
        Manivannan Sadhasivam <mani@...nel.org>,
        Grzegorz Szymaszek <gszymaszek@...rt.pl>
Subject: Re: [PATCH 1/3] dt-bindings: mmc: mmci: add a property to disable DMA LLI

On Thu, 17 Mar 2022 at 11:18, Yann Gautier <yann.gautier@...s.st.com> wrote:
>
> On 3/17/22 11:00, Ulf Hansson wrote:
> > On Tue, 15 Mar 2022 at 09:26, Yann Gautier <yann.gautier@...s.st.com> wrote:
> >>
> >> On 3/14/22 23:43, Linus Walleij wrote:
> >>> "On Fri, Mar 4, 2022 at 2:52 PM Yann Gautier <yann.gautier@...s.st.com> wrote:
> >>>
> >>>> On STMicroelectronics variant of PL18x, the DMA Linked Lists are supported
> >>>> starting from revision v2 of the peripheral. But it has limitations,
> >>>> as all the buffers should be aligned on block size (except the last one).
> >>>> But this cannot be guaranteed with SDIO. We should then have a property
> >>>> to disable the support of LLI.
> >>>>
> >>>> Signed-off-by: Yann Gautier <yann.gautier@...s.st.com>
> >>>
> >>> Actually I think this is present also on the ux500 variants. See:
> >>> commit 2253ed4b36dc876d1598c4dab5587e537ec68c34
> >>> "mmc: mmci: Support any block sizes for ux500v2 and qcom variant"
> >>>
> >>> Spot the variant data "dma_power_of_2".
> >>>
> >>> So whatever property you add
> >>> to the variant data (not in the device tree please) should
> >>> be added to the ux500 variants as well, it will *VERY* likely
> >>> have a problem with LLI elements not being a power of 2
> >>> as it is the ancestor of later STMicro variants.
> >>>
> >>> It might actually be the reason for some annoying WiFi error
> >>> messages I have seen :/
> >>>
> >>> Yours,
> >>> Linus Walleij
> >>
> >> Hi Linus,
> >>
> >> The STM32 variant uses an internal DMA, and the DMA functions are in its
> >> dedicated file. So I was planning to do the same as what is done in
> >> meson-gx-mmc.c: using a bounce buffer to copy from/to in case DMA
> >> constraints are not fulfilled. Not sure it can help for Ux500.
> >
>
> Hi Ulf,
>
> > We already have a bounce buffer in mmci_pio_read(), but we need one in
> > mmc_pio_write() too, which hasn't been implemented yet.
>
> The idea is to keep using our internal DMA, and not switch to pio mode.

Okay, but doesn't that become awfully inefficient, especially if we
end up with several smaller sg-list-elements.

The switch to pio would only be temporary for the particular request
that has "bogus" buffers.

>
> >
> >>
> >> Ulf, before I send my new series (although it is not ready yet), would
> >> you be OK with the bounce buffer idea?
> >
> > Yes, that works for me.
> I have patches almost ready, I'll send that soon.

Okay!

>
> >
> > On the other hand, it would be even better if we could specify the
> > buffer limitations per mmc host instance, so upper layers (SDIO func
> > drivers) could conform to these - and use better buffers, to achieve a
> > better performance.
>
> We've seen things that could be improved in the wifi driver. I'll check
> what could be done.

Great!

Kind regards
Uffe