linux-kernel - Re: [PATCH v4 6/6] firmware: tegra: bpmp: Add support for DRAM MRQ GSCs

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Date:   Thu, 8 Jun 2023 14:22:23 +0300
From:   Peter De Schrijver <pdeschrijver@...dia.com>
To:     Thierry Reding <thierry.reding@...il.com>
CC:     <jonathanh@...dia.com>, <mperttunen@...dia.com>,
        <sudeep.holla@....com>, <talho@...dia.com>, <robh@...nel.org>,
        <linux-tegra@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <stefank@...dia.com>, <krzysztof.kozlowski@...aro.org>
Subject: Re: [PATCH v4 6/6] firmware: tegra: bpmp: Add support for DRAM MRQ
 GSCs

On Wed, Jun 07, 2023 at 05:57:39PM +0200, Thierry Reding wrote:
> > No, on the contrary, now it's clear you can either have void __iomem *
> > and struct gen_pool * or void *virt but not both.
> 
> No, it's not clear. You can have one part of your driver write the
> sram.virt field and another read dram.virt and they'll end up pointing
> at the same memory location but with different meaning. That's why you
> need to introduce the enumeration in order to specify which one of the
> two you want to pick.
> 
> And that's exactly where you start introducing the potential for
> inconsistency: now you need to be extra careful that the enumeration and
> the unions are set correctly. You effectively have two sources of truth
> and they don't necessarily match. You can also end up (at least
> theoretically) with the invalid value, so you need an extra check for
> that too.
> 
> You can avoid all of those inconsistencies if you reduce this to one
> source of truth, namely the pointers that you're going to use.
> 

There are 4 possible states for these pointers:
both NULL
both non-NULL
sram pointer NULL, dram pointer non-NULL
dram pointer NULL, sram pointer non-NULL

So how is this one source of truth?

Peter.