linux-kernel - Re: [PATCH] drm: make drm_get_format

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Date:	Mon, 15 Aug 2016 16:07:46 +0100
From:	Eric Engestrom <eric.engestrom@...tec.com>
To:	Jani Nikula <jani.nikula@...ux.intel.com>,
	Tom St Denis <tom.stdenis@....com>,
	<intel-gfx@...ts.freedesktop.org>,
	Eric Engestrom <eric@...estrom.ch>,
	Michel Dänzer <michel.daenzer@....com>,
	<linux-kernel@...r.kernel.org>, <dri-devel@...ts.freedesktop.org>,
	Xinliang Liu <z.liuxinliang@...ilicon.com>,
	David Zhang <david1.zhang@....com>,
	Wei Yongjun <yongjun_wei@...ndmicro.com.cn>,
	Vitaly Prosyak <vitaly.prosyak@....com>,
	Daniel Vetter <daniel.vetter@...el.com>,
	Junwei Zhang <Jerry.Zhang@....com>,
	Alex Deucher <alexander.deucher@....com>,
	Flora Cui <Flora.Cui@....com>,
	Gustavo Padovan <gustavo.padovan@...labora.co.uk>,
	Christian König <christian.koenig@....com>
Subject: Re: [PATCH] drm: make drm_get_format_name thread-safe

On Mon, Aug 15, 2016 at 03:52:07PM +0200, Daniel Vetter wrote:
> On Mon, Aug 15, 2016 at 04:13:54PM +0300, Jani Nikula wrote:
> > On Mon, 15 Aug 2016, Eric Engestrom <eric.engestrom@...tec.com> wrote:
> > > On Mon, Aug 15, 2016 at 12:54:01PM +0300, Jani Nikula wrote:
> > >> On Mon, 15 Aug 2016, Eric Engestrom <eric@...estrom.ch> wrote:
> > >> > Signed-off-by: Eric Engestrom <eric@...estrom.ch>
> > >> > ---
> > >> >
> > >> > I moved the main bits to be the first diffs, shouldn't affect anything
> > >> > when applying the patch, but I wanted to ask:
> > >> > I don't like the hard-coded `32` the appears in both kmalloc() and
> > >> > snprintf(), what do you think? If you don't like it either, what would
> > >> > you suggest? Should I #define it?
> > >> >
> > >> > Second question is about the patch mail itself: should I send this kind
> > >> > of patch separated by module, with a note requesting them to be squashed
> > >> > when applying? It has to land as a single patch, but for review it might
> > >> > be easier if people only see the bits they each care about, as well as
> > >> > to collect ack's/r-b's.
> > >> >
> > >> > Cheers,
> > >> >   Eric
> > >> >
> > >> > ---
> > >> >  drivers/gpu/drm/amd/amdgpu/dce_v10_0.c          |  6 ++--
> > >> >  drivers/gpu/drm/amd/amdgpu/dce_v11_0.c          |  6 ++--
> > >> >  drivers/gpu/drm/amd/amdgpu/dce_v8_0.c           |  6 ++--
> > >> >  drivers/gpu/drm/drm_atomic.c                    |  5 ++--
> > >> >  drivers/gpu/drm/drm_crtc.c                      | 21 ++++++++-----
> > >> >  drivers/gpu/drm/drm_fourcc.c                    | 17 ++++++-----
> > >> >  drivers/gpu/drm/hisilicon/kirin/kirin_drm_ade.c |  6 ++--
> > >> >  drivers/gpu/drm/i915/i915_debugfs.c             | 11 ++++++-
> > >> >  drivers/gpu/drm/i915/intel_atomic_plane.c       |  6 ++--
> > >> >  drivers/gpu/drm/i915/intel_display.c            | 39 ++++++++++++++++---------
> > >> >  drivers/gpu/drm/radeon/atombios_crtc.c          | 12 +++++---
> > >> >  include/drm/drm_fourcc.h                        |  2 +-
> > >> >  12 files changed, 89 insertions(+), 48 deletions(-)
> > >> >
> > >> > diff --git a/drivers/gpu/drm/drm_fourcc.c b/drivers/gpu/drm/drm_fourcc.c
> > >> > index 0645c85..38216a1 100644
> > >> > --- a/drivers/gpu/drm/drm_fourcc.c
> > >> > +++ b/drivers/gpu/drm/drm_fourcc.c
> > >> > @@ -39,16 +39,14 @@ static char printable_char(int c)
> > >> >   * drm_get_format_name - return a string for drm fourcc format
> > >> >   * @format: format to compute name of
> > >> >   *
> > >> > - * Note that the buffer used by this function is globally shared and owned by
> > >> > - * the function itself.
> > >> > - *
> > >> > - * FIXME: This isn't really multithreading safe.
> > >> > + * Note that the buffer returned by this function is owned by the caller
> > >> > + * and will need to be freed.
> > >> >   */
> > >> >  const char *drm_get_format_name(uint32_t format)
> > >> 
> > >> I find it surprising that a function that allocates a buffer returns a
> > >> const pointer. Some userspace libraries have conventions about the
> > >> ownership based on constness.
> > >> 
> > >> (I also find it suprising that kfree() takes a const pointer; arguably
> > >> that call changes the memory.)
> > >> 
> > >> Is there precedent for this?
> > >> 
> > >> BR,
> > >> Jani.
> > >
> > > It's not a const pointer, it's a normal pointer to a const char, i.e.
> > > you can do as you want with the pointer but you shouldn't change the
> > > chars it points to.
> > 
> > Ermh, that's what I meant even if I was sloppy in my reply. And arguably
> > freeing the bytes the pointer points at changes them, albeit subtly. And
> > having a function return a pointer to const data is often an indication
> > that the ownership of the data isn't transfered, i.e. you're not
> > supposed to free it yourself.
> 
> I already applied the patch, but yes dropping the const would be a good
> hint to callers that they now own that block of memory. Eric, can you pls
> follow up with a fix up patch - drm-misc is non-rebasing?
> -Daniel

I didn't know about that convention. I'll send a fixup patch, but it'll
have to wait until tomorrow night. I hope that's not an issue :(

Cheers,
  Eric