From: Jyri Sarha Subject: Re: [PATCH v6 1/7] drm/tilcdc: ensure nonatomic iowrite64 is not used Date: Sat, 5 Aug 2017 16:30:35 +0300 Message-ID: References: <20170803183018.5889-1-logang@deltatee.com> <20170803183018.5889-2-logang@deltatee.com> <2b41b02c-07e3-b927-f276-a29480f94285@ti.com> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Cc: Arnd Bergmann , Greg Kroah-Hartman , Andy Shevchenko , =?UTF-8?Q?Horia_Geant=c4=83?= , Stephen Bates , David Airlie To: Tomi Valkeinen , Logan Gunthorpe , , , , Return-path: In-Reply-To: <2b41b02c-07e3-b927-f276-a29480f94285@ti.com> Content-Language: en-GB Sender: linux-arch-owner@vger.kernel.org List-Id: linux-crypto.vger.kernel.org On 08/04/17 16:03, Tomi Valkeinen wrote: > On 03/08/17 21:30, Logan Gunthorpe wrote: >> Add a check to ensure iowrite64 is only used if it is atomic. >> >> It was decided in [1] that the tilcdc driver should not be using an >> atomic operation (so it was left out of this patchset). However, it turns >> out that through the drm code, a nonatomic header is actually included: >> >> include/linux/io-64-nonatomic-lo-hi.h >> is included from include/drm/drm_os_linux.h:9:0, >> from include/drm/drmP.h:74, >> from include/drm/drm_modeset_helper.h:26, >> from include/drm/drm_atomic_helper.h:33, >> from drivers/gpu/drm/tilcdc/tilcdc_crtc.c:19: >> >> And thus, without this change, this patchset would inadvertantly >> change the behaviour of the tilcdc driver. > > I haven't really followed the discussion on this, but if the tilcdc's > use of iowrite64 causes (real) problems/complications elsewhere, I think > we could drop it. > > The problem is that the HW has a race issue, and the two registers in > question should be written as close to each other as possible. We > thought a single 64bit write, writing to both registers in one go, would > improve that slightly, compared to two 32 bit writes. > > Jyri, correct me if I'm wrong, but we have no proof that it actually > helps, and it might be that even if it helps, the difference is > theoretical. Probably if we ensure the irqs are off when we do two 32 > bit writes, we're already close enough to the optimal case. > For the sake of this particular case you are right, the atomicity is probably not that important here. But in general ARM7 has an atomic 64bit write and I think it is a shame if it can not be easily used in linux kernel. Best regards, Jyri