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

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Message-ID: <0bc53018-fce4-4104-fa47-6e60d2367d69@foss.st.com>
Date:   Tue, 15 Mar 2022 09:26:01 +0100
From:   Yann Gautier <yann.gautier@...s.st.com>
To:     Linus Walleij <linus.walleij@...aro.org>
CC:     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>,
        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 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.

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