linux-kernel - Re: [PATCH] drm/amd/pm: replace 1-element arrays with flexible-array members

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Date:   Thu, 9 Nov 2023 10:06:38 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     José Pekkarinen <jose.pekkarinen@...hound.fi>
Cc:     evan.quan@....com, alexander.deucher@....com,
        christian.koenig@....com, Xinhui.Pan@....com,
        skhan@...uxfoundation.org, airlied@...il.com, daniel@...ll.ch,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, linux-kernel-mentees@...ts.linux.dev
Subject: Re: [PATCH] drm/amd/pm: replace 1-element arrays with flexible-array
 members

On Thu, Nov 09, 2023 at 10:43:50AM +0200, José Pekkarinen wrote:
> On 2023-11-08 09:29, Greg KH wrote:
> > On Wed, Nov 08, 2023 at 08:54:35AM +0200, José Pekkarinen wrote:
> > > The following case seems to be safe to be replaced with a flexible
> > > array
> > > to clean up the added coccinelle warning. This patch will just do it.
> > > 
> > > drivers/gpu/drm/amd/pm/powerplay/smumgr/smu8_smumgr.h:76:38-63:
> > > WARNING use flexible-array member instead (https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays)
> > > 
> > > Signed-off-by: José Pekkarinen <jose.pekkarinen@...hound.fi>
> > > ---
> > >  drivers/gpu/drm/amd/pm/powerplay/smumgr/smu8_smumgr.h | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/gpu/drm/amd/pm/powerplay/smumgr/smu8_smumgr.h
> > > b/drivers/gpu/drm/amd/pm/powerplay/smumgr/smu8_smumgr.h
> > > index c7b61222d258..1ce4087005f0 100644
> > > --- a/drivers/gpu/drm/amd/pm/powerplay/smumgr/smu8_smumgr.h
> > > +++ b/drivers/gpu/drm/amd/pm/powerplay/smumgr/smu8_smumgr.h
> > > @@ -73,7 +73,7 @@ struct smu8_register_index_data_pair {
> > > 
> > >  struct smu8_ih_meta_data {
> > >  	uint32_t command;
> > > -	struct smu8_register_index_data_pair register_index_value_pair[1];
> > > +	struct smu8_register_index_data_pair register_index_value_pair[];
> > 
> > Did you just change this structure size without any need to change any
> > code as well?  How was this tested?
> 
>     I didn't find any use of that struct member, if I missed
> something here, please let me know and I'll happily address any
> needed further work.

I don't think this is even a variable array.  It's just a one element
one, which is fine, don't be confused by the coccinelle "warning" here,
it's fired many false-positives and you need to verify this properly
with the driver authors first before changing anything.

In short, you just changed the size of this structure, are you _sure_
you can do that?  And yes, it doesn't look like this field is used, but
the structure is, so be careful.

thanks,

greg k-h