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

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Message-ID: <20220329153114.GA58120@thinkpad>
Date:   Tue, 29 Mar 2022 21:01:14 +0530
From:   Manivannan Sadhasivam <mani@...nel.org>
To:     Yann Gautier <yann.gautier@...s.st.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Ulf Hansson <ulf.hansson@...aro.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,
        Grzegorz Szymaszek <gszymaszek@...rt.pl>
Subject: Re: [PATCH 1/3] dt-bindings: mmc: mmci: add a property to disable
 DMA LLI

On Tue, Mar 15, 2022 at 09:26:01AM +0100, Yann Gautier 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.
> 

Irrelevant to this patch: May I know why the internal DMA cannot be represented
as a dmaengine driver? We started seeing these internal DMA implementations in
the other subsystems as well with pointers towards MMC core [1].

Thanks,
Mani

[1] https://lore.kernel.org/all/CA+V-a8tfUgvzPyMe_FHuz=8mmC6dPHP7E=e+nCzOey04vCcAkg@mail.gmail.com/

> Ulf, before I send my new series (although it is not ready yet), would you
> be OK with the bounce buffer idea?
> 
> 
> Best regards,
> Yann