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Miller" , Udit Tiwari , Md Sadre Alam , Dmitry Baryshkov , Manivannan Sadhasivam , Bjorn Andersson , Mukesh Kumar Savaliya , Peter Ujfalusi , Michal Simek , Frank Li , Neil Armstrong , Vignesh Raghavendra , dmaengine@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-crypto@vger.kernel.org, linux-arm-kernel@lists.infradead.org, sashiko-reviews@lists.linux.dev, Bartosz Golaszewski Subject: Re: [PATCH v23 06/14] dmaengine: qcom: bam_dma: add support for BAM locking Message-ID: References: <20260721-qcom-qce-cmd-descr-v23-0-1be02deffda9@oss.qualcomm.com> <20260721-qcom-qce-cmd-descr-v23-6-1be02deffda9@oss.qualcomm.com> <20260721134853.2B2401F000E9@smtp.kernel.org> Precedence: bulk X-Mailing-List: linux-arm-msm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Wed, Jul 22, 2026 at 02:34:52AM -0700, Bartosz Golaszewski wrote: > On Wed, 22 Jul 2026 10:59:09 +0200, Stephan Gerhold > said: > > On Tue, Jul 21, 2026 at 01:48:52PM +0000, sashiko-bot@kernel.org wrote: > >> Thank you for your contribution! Sashiko AI review found 5 potential issue(s) to consider: > >> - [High] State leak causing hardware misconfiguration on channel reuse due to uncleared `lock_scratchpad_addr` and hardcoded direction. > >> - [High] Channel state corruption when `bam_slave_config` fails validation. > >> - [High] Infinite BH workqueue loop and unratelimited log flood on memory allocation failure. > >> - [High] DMA API violation: `lock_ce` buffer shares a cacheline with frequently CPU-modified struct members. > >> - [Low] Unused assignment of `hdesc` in IRQ handler. > >> -- > >> > >> --- Patch [6]: [PATCH v23 06/14] dmaengine: qcom: bam_dma: add support for BAM locking --- > >> commit 8b44c5e4935cbb61832adb3262df00b3b606a283 > >> Author: Bartosz Golaszewski > >> > >> dmaengine: qcom: bam_dma: add support for BAM locking > >> > >> This commit adds support for BAM pipe locking by prepending and appending > >> dummy command descriptors with LOCK/UNLOCK bits when starting DMA on a TX > >> channel. It uses a peripheral-local scratchpad address communicated via slave > >> config for the dummy writes. > >> > >> > diff --git a/drivers/dma/qcom/bam_dma.c b/drivers/dma/qcom/bam_dma.c > >> > index f3e713a5259c2..411c60c4433e2 100644 > >> > --- a/drivers/dma/qcom/bam_dma.c > >> > +++ b/drivers/dma/qcom/bam_dma.c > [...] > >> [ ... ] > >> > @@ -1064,6 +1197,7 @@ static void bam_start_dma(struct bam_chan *bchan) > >> > > >> > lockdep_assert_held(&bchan->vc.lock); > >> > > >> > + vd = vchan_next_desc(&bchan->vc); > >> > if (!vd) > >> > return; > >> > > >> > @@ -1072,6 +1206,24 @@ static void bam_start_dma(struct bam_chan *bchan) > >> > return; > >> > > >> > while (vd && !IS_BUSY(bchan)) { > >> > + /* > >> > + * Open a LOCK/UNLOCK bracket around each fresh sequence. > >> > + * Sentinels inserted by bam_setup_pipe_lock() are skipped: they > >> > + * already have bam_locked set and must not trigger a second pair. > >> > + */ > >> > + if (!bchan->bam_locked) { > >> > + ret = bam_setup_pipe_lock(bchan); > >> > + if (ret) { > >> > + dev_err_ratelimited(bdev->dev, > >> > + "failed to setup the pipe lock, deferring transfer: %d\n", > >> > + ret); > >> > + queue_work(system_bh_highpri_wq, &bdev->work); > >> > + break; > >> > + } > >> > >> [Severity: High] > >> Does requeuing to a high-priority BH workqueue upon ENOMEM cause CPU > >> starvation? Because system_bh_highpri_wq executes in softirq context, > >> requeuing immediately without sleeping prevents the memory reclaim needed > >> to resolve the GFP_NOWAIT failure, leading to an infinite spin loop. > >> > > > > It looks like my suggestion to queue_work() again in the error path of > > bam_start_dma() wasn't great if we end up causing an infinite spin loop. > > We do need to retry somehow or report an error though, I don't think > > aborting and leaving the desccriptors completely unhandled is an option > > either... :( > > > > One option would be to try to avoid the allocation and write the lock > > descriptors directly into the FIFO, but this will probably get really > > messy as well, since you would need to carefully modify the FIFO > > management in several functions ... > > > > I don't have a good spontaenous idea how to solve this right now. :/ > > > > Pre-allocate the lock descriptors (if needed) in bam_prep_slave_sg()? As in: > call bam_make_lock_desc() in bam_prep_slave_sg(), store the descriptors and > make bam_setup_pipe_lock() return void as it will no longer be possible for it > to fail? It would just grab the preallocated descriptors. > I considered suggesting this, but how do you know which descriptor needs it in bam_prep_slave_sg()? It's just the allocation, afaict it doesn't tell you anything about the order in which they will be submitted. :/ We could always allocate the extra lock descriptors and waste the extra memory for the descriptors that won't need it. That should work, but is also not great ... I was hammering at the code a bit earlier to try the "write the lock descriptors directly into the FIFO" approach. It has some advantages, but also makes the FIFO management quite a bit trickier, hard to get it right without full concentration. :') I _think_ it could potentially work, but if you have a simpler idea I'm also happy to throw away the mess I started. :-) Thanks, Stephan