linux-kernel - Re: [PATCH 2/5] staging: vc04_services: Remove cache-line-size property.

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Message-ID: <51dc71d7-e8f5-2785-ad13-a941bc9278ab@raspberrypi.org>
Date:   Wed, 7 Mar 2018 12:39:56 +0000
From:   Phil Elwell <phil@...pberrypi.org>
To:     Stefan Wahren <stefan.wahren@...e.com>,
        Rob Herring <robh+dt@...nel.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        Eric Anholt <eric@...olt.net>,
        Mark Rutland <mark.rutland@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        devicetree@...r.kernel.org
Cc:     linux-rpi-kernel@...ts.infradead.org,
        bcm-kernel-feedback-list@...adcom.com,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/5] staging: vc04_services: Remove cache-line-size
 property.

On 07/03/2018 12:10, Stefan Wahren wrote:
> Hi Phil,

<snip>

>> It is the L2 cache line size that matters, but as long as you end up with
>> the numbers Stefan mentioned - 32 on BCM2835, 64 on BCM2836 and BCM2837 -
>> I'm not too bothered how you get there.
> 
> i think a kernel with bcm2835_defconfig on RPi 2 could be such a corncase.
> 
> Am i right that the firmware doesn't rely on the existence of "cache-line-size"?

Because of the way partial cache lines are handled it is more important that the
two sides agree than that the value is correct. As a result, the firmware treats
the absence of a "cache_line_size" DT parameter (that sets the "cache-line-size"
property) in the DTB as an indication that the kernel driver pre-dates the ability
to switch, and uses the old fixed value of 32 as a fallback. Otherwise it sets the
parameter and the internal value used by the VPU-side VCHIQ to the correct value.

There are a number of ways to fix this, the easiest of which is to assume that the kernel
driver will either read the property or be able to work out the correct value, so
the VPU should always use the correct value regardless of the success of applying
the parameter/changing the property.

> Btw it would be nice to get fixed the corruption on ARM64 [1].

This is almost certainly due to the logic described above.

> 
> Stefan
> 
> [1] - https://github.com/lategoodbye/rpi-zero/issues/23
> 
>>
>> Phil
>>
>> _______________________________________________
>> linux-arm-kernel mailing list
>> linux-arm-kernel@...ts.infradead.org
>> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

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