linux-kernel - Re: [RFC PATCH 5/8] qaic: Implement data path

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Date:   Thu, 14 May 2020 18:37:34 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Jeffrey Hugo <jhugo@...eaurora.org>
Cc:     arnd@...db.de, manivannan.sadhasivam@...aro.org,
        bjorn.andersson@...aro.org, wufan@...eaurora.org,
        pratanan@...eaurora.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 5/8] qaic: Implement data path

On Thu, May 14, 2020 at 10:12:03AM -0600, Jeffrey Hugo wrote:
> On 5/14/2020 9:56 AM, Greg KH wrote:
> > On Thu, May 14, 2020 at 09:06:53AM -0600, Jeffrey Hugo wrote:
> > > On 5/14/2020 8:14 AM, Greg KH wrote:
> > > > On Thu, May 14, 2020 at 08:07:43AM -0600, Jeffrey Hugo wrote:
> > > > > +struct qaic_execute {
> > > > > +	__u16		ver;    /* struct version, must be 1 */
> > > > 
> > > > No need for structures to be versioned.  If you change something, then
> > > > add a new ioctl if you really needed it.
> > > 
> > > Huh.  We had thought the botching ioctls document advised having a version,
> > > but as I double check that document, it infact does not.
> > > 
> > > Will remove.
> > 
> > Thanks, you can also remove the "reserved" variables as well as those
> > will not be needed either.
> 
> Are you sure?
> 
> Documentation/process/botching-up-ioctls.rst
> Starting at Line 38:
> 
> "Pad the entire struct to a multiple of 64-bits if the structure contains
> 64-bit types - the structure size will otherwise differ on 32-bit versus
> 64-bit. Having a different structure size hurts when passing arrays of
> structures to the kernel, or if the kernel checks the structure size, which
> e.g. the drm core does."
> 
> The "reserved" variables seem to be in line with that.

Padding is fine to use, but don't use that as a "I'm reserving this to
use it for later" type of thing which is how I read the structure
definitions.  I might be totally wrong, but you should be explicit here.

thanks,

greg k-h