linux-kernel - Re: [Patch V2] memory: tegra: Remove clients SID override programming

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Message-ID: <2a27c32d-b013-e914-62ae-393af3987716@linaro.org>
Date:   Thu, 24 Nov 2022 12:34:43 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Ashish Mhetre <amhetre@...dia.com>, thierry.reding@...il.com,
        jonathanh@...dia.com, dmitry.osipenko@...labora.com,
        linux-kernel@...r.kernel.org, linux-tegra@...r.kernel.org
Subject: Re: [Patch V2] memory: tegra: Remove clients SID override programming

On 24/11/2022 11:20, Ashish Mhetre wrote:
> 
> On 11/22/2022 4:44 PM, Krzysztof Kozlowski wrote:
>> External email: Use caution opening links or attachments
>>
>>
>> On 22/11/2022 06:31, Ashish Mhetre wrote:
>>> On newer Tegra releases, early boot SID override programming and SID
>>> override programming during resume is handled by bootloader.
>>> Also, SID override is programmed on-demand during probe_finalize() call
>>> of IOMMU which is done in tegra186_mc_client_sid_override() in this same
>>> file. This function does it more correctly by checking if write is
>>> permitted on SID override register. It also checks if SID override
>>> register is already written with correct value and skips re-writing it
>>> in that case.
>>> Hence, removing the SID override programming of all clients.
>>>
>>> Fixes: 393d66fd2cac ("memory: tegra: Implement SID override programming")
>> I could not get from commit msg what is the bug being fixed. You just
>> said "more correctly", but usually things are either correct or not.
>> What are visible effects of the bug?
>>
>> Otherwise it sounds more like optimization or a bit better approach, but
>> not a bugfix.
>>
>> Best regards,
>> Krzysztof
> 
> Thanks for the review. In the function tegra186_mc_program_sid() which is
> getting removed, SID override register of all clients is written without
> checking if secure firmware has allowed write on it or not. If write is
> disabled by secure firmware then it can lead to errors coming from secure
> firmware and hang in kernel boot. So, that's a possible bug.

Please add it to commit msg, because it justifies Fixes tag and probably
backport.

> Also, it's an optimization over current approach because it saves time by
> removing re-writing of these SID override registers as in new Tegra releases
> SID override of all clients is programmed by bootloader. So, MC driver don't
> need to program them again.

Best regards,
Krzysztof