linux-kernel - Re: [PATCH v3 2/3] soc: qcom: rmtfs: Support discarding guard pages

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Message-ID: <j5ivvbjprgdcekujzjiobk7gpoaz3jxpnda5dsgoh6f4hsn5id@3ug5i74ubkqf>
Date:   Thu, 21 Sep 2023 19:51:42 -0700
From:   Bjorn Andersson <andersson@...nel.org>
To:     Stephan Gerhold <stephan@...hold.net>
Cc:     Bjorn Andersson <quic_bjorande@...cinc.com>,
        Andy Gross <agross@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/3] soc: qcom: rmtfs: Support discarding guard pages

On Thu, Sep 21, 2023 at 08:04:06PM +0200, Stephan Gerhold wrote:
> On Wed, Sep 20, 2023 at 07:37:31PM -0700, Bjorn Andersson wrote:
> > In some configurations, the exact placement of the rmtfs shared memory
> > region isn't so strict. The DeviceTree author can then choose to use the
> > "size" property and rely on the OS for placement (in combination with
> > "alloc-ranges", if desired).
> > 
> > But on some platforms the rmtfs memory region may not be allocated
> > adjacent to regions allocated by other clients. Add support for
> > discarding the first and last 4k block in the region, if
> > qcom,use-guard-pages is specified in DeviceTree.
> > 
> > Signed-off-by: Bjorn Andersson <quic_bjorande@...cinc.com>
> > ---
> >  drivers/soc/qcom/rmtfs_mem.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> > 
> > diff --git a/drivers/soc/qcom/rmtfs_mem.c b/drivers/soc/qcom/rmtfs_mem.c
> > index f83811f51175..83bba9321e72 100644
> > --- a/drivers/soc/qcom/rmtfs_mem.c
> > +++ b/drivers/soc/qcom/rmtfs_mem.c
> > @@ -200,6 +200,15 @@ static int qcom_rmtfs_mem_probe(struct platform_device *pdev)
> >  	rmtfs_mem->client_id = client_id;
> >  	rmtfs_mem->size = rmem->size;
> >  
> > +	/*
> > +	 * If requested, discard the first and last 4k block in order to ensure
> > +	 * that the rmtfs region isn't adjacent to other protected regions.
> > +	 */
> > +	if (of_property_present(node, "qcom,use-guard-pages")) {
> 
> I think of_property_read_bool() would be more fitting here. Right now
> of_property_present() is just a wrapper around of_property_read_bool().
> Semantically reading a bool fits better here though. :-)
> 

Are you saying that you would prefer this to be a bool, so hat you can
give it a "false" value? Or you are simply saying "it walks like a
boolean, quacks like a boolean, let's use the boolean accessor"?

> Feel free to fix that up while applying.
> 
> FWIW I don't really have an opinion if "qcom,use-guard-pages" is a good
> way to describe this in the DT. For the implementation side feel free to
> add my
> 

Right, I don't think I commented on your suggestion to make the size of
the guard page configurable. I am not aware of any current or upcoming
reasons for adding such complexity, so I'd simply prefer to stick with a
boolean. Should that need arise, I think this model would allow
extension to express that.

Regards,
Bjorn

> Reviewed-by: Stephan Gerhold <stephan@...hold.net>
> 
> Thanks,
> Stephan
> 
> > +		rmtfs_mem->addr += SZ_4K;
> > +		rmtfs_mem->size -= 2 * SZ_4K;
> > +	}
> > +
> >  	device_initialize(&rmtfs_mem->dev);
> >  	rmtfs_mem->dev.parent = &pdev->dev;
> >  	rmtfs_mem->dev.groups = qcom_rmtfs_mem_groups;
> > 
> > -- 
> > 2.25.1
> >