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

From: Yann Gautier <yann.gautier@foss.st.com>
To: Linus Walleij <linus.walleij@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>,
	Ulf Hansson <ulf.hansson@linaro.org>,
	<devicetree@vger.kernel.org>,
	<linux-arm-kernel@lists.infradead.org>,
	<linux-kernel@vger.kernel.org>, <linux-mmc@vger.kernel.org>,
	<linux-stm32@st-md-mailman.stormreply.com>,
	Alexandre Torgue <alexandre.torgue@foss.st.com>,
	Christophe Kerello <christophe.kerello@foss.st.com>,
	Ludovic Barre <ludovic.barre@foss.st.com>,
	Maxime Coquelin <mcoquelin.stm32@gmail.com>,
	Philipp Zabel <p.zabel@pengutronix.de>,
	Russell King <linux@armlinux.org.uk>, Marek Vasut <marex@denx.de>,
	<kernel@dh-electronics.com>,
	Manivannan Sadhasivam <mani@kernel.org>,
	Grzegorz Szymaszek <gszymaszek@short.pl>
Subject: Re: [PATCH 1/3] dt-bindings: mmc: mmci: add a property to disable DMA LLI
Date: Tue, 15 Mar 2022 09:26:01 +0100	[thread overview]
Message-ID: <0bc53018-fce4-4104-fa47-6e60d2367d69@foss.st.com> (raw)
In-Reply-To: <CACRpkdYQz+-im3n-r0_8RKL7so2bHS=aZobty4BbzixmPzms-Q@mail.gmail.com>

On 3/14/22 23:43, Linus Walleij wrote:
> "On Fri, Mar 4, 2022 at 2:52 PM Yann Gautier <yann.gautier@foss.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@foss.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