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

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Message-ID: <b59fd0f0-cedd-dc48-52bf-090358ce7786@linaro.org>
Date:   Wed, 30 Mar 2022 11:25:30 +0100
From:   Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
To:     Linus Walleij <linus.walleij@...aro.org>,
        Manivannan Sadhasivam <mani@...nel.org>
Cc:     Yann Gautier <yann.gautier@...s.st.com>, ulf.hansson@...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,
        Grzegorz Szymaszek <gszymaszek@...rt.pl>
Subject: Re: [PATCH 1/3] dt-bindings: mmc: mmci: add a property to disable DMA
 LLI



On 29/03/2022 22:16, Linus Walleij wrote:
> On Tue, Mar 29, 2022 at 7:33 PM Manivannan Sadhasivam <mani@...nel.org> wrote:
>> [Yann]
>>> As for Prabhakar's answer, the IDMA here is inside our IP, and not used in
>>> any other IP. So I'm not sure it is really relevant to move that to another
>>> dmaengine driver.
>>
>> Okay, I think this justification makes sense. I was worried of DMA IPs that get
>> sandwiched into many peripherals like the one on Renesas platforms. It turned
>> out that each subsystem has to add internal DMA support for it :/
> 
> That is a justified worry.
> 
> Qualcomm has "BAM DMA" (I think it is called?) which is added to each IP
> that needs DMA. drivers/mmc/host/mmci_qcom_dml.c
> It's for older Qualcomm platforms but I *think* it is actually not just used
> for the MMCI, just noone ever got around to adding it to any other
> peripheral? Srini do you know?

There are multiple instances of BAM (Bus access manager) on Qcom SoC, 
some of these instances are dedicated for each peripheral instance.
In this particular case we have 4 instances of BAM each of which are 
dedicated to 4 instances of SD Controllers.

BAM dmaengine is used across many Qualcomm peripheral drivers.

--srini

> 
> Yours,
> Linus Walleij