From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 27F2EC4332F for ; Fri, 18 Nov 2022 09:20:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S241805AbiKRJUc (ORCPT ); Fri, 18 Nov 2022 04:20:32 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38026 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S241877AbiKRJUV (ORCPT ); Fri, 18 Nov 2022 04:20:21 -0500 Received: from perceval.ideasonboard.com (perceval.ideasonboard.com [213.167.242.64]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9A78BAE49 for ; Fri, 18 Nov 2022 01:20:20 -0800 (PST) Received: from pyrite.rasen.tech (h175-177-042-159.catv02.itscom.jp [175.177.42.159]) by perceval.ideasonboard.com (Postfix) with ESMTPSA id 1F588AF4; Fri, 18 Nov 2022 10:20:16 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=ideasonboard.com; s=mail; t=1668763219; bh=J3I6RUlRxdezXLwMbRikvm0hGh3t2VGEDOt6b5txUq4=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=pMWpEJCOEyZBYRRsNMiZg8zZG70/Y9I2DVC9Gpa6SVbSfZclTPp4qInZhRqGDSh1N ARh2HI++7kPUwSARH9Wm/kBZn2MJRM468e+q3py/qwT5shCrt7STTII3vKS6FJ4Jtl xrdgYDtPPfX63RwSodap+21gILuYyg5eT3dGSL68= Date: Fri, 18 Nov 2022 18:20:10 +0900 From: Paul Elder To: Laurent Pinchart Cc: linux-media@vger.kernel.org, Rui Miguel Silva , Steve Longerbeam , Philipp Zabel , kernel@pengutronix.de, linux-imx@nxp.com Subject: Re: [PATCH v3] media: imx: imx7-media-csi: Add support for fast-tracking queued buffers Message-ID: References: <20220908083739.11586-1-laurent.pinchart@ideasonboard.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220908083739.11586-1-laurent.pinchart@ideasonboard.com> Precedence: bulk List-ID: X-Mailing-List: linux-media@vger.kernel.org Gentle ping Paul On Thu, Sep 08, 2022 at 11:37:39AM +0300, Laurent Pinchart wrote: > From: Paul Elder > > The CSI hardware compatible with this driver handles buffers using a > ping-pong mechanism with two sets of destination addresses. Normally, > when an interrupt comes in to signal the completion of one buffer, say > FB1, it assigns the next buffer in the queue to the next FB1, and the > hardware starts to capture into FB2 in the meantime. > > In a buffer underrun situation, in the above example without loss of > generality, if a new buffer is queued before the interrupt for FB1 comes > in, we can program the buffer into FB2 (which is programmed with a dummy > buffer, as there is a buffer underrun). > > This of course races with the interrupt that signals FB1 completion, as > once that interrupt comes in, we are no longer guaranteed that the > programming of FB2 was in time and must assume it was too late. This > race is resolved partly by locking the programming of FB2. If it came > after the interrupt for FB1, then the variable that is used to determine > which FB to program would have been swapped by the interrupt handler. > > This alone isn't sufficient, however, because the interrupt could still > be generated (thus the hardware starts capturing into the other fb) > while the fast-tracking routine has the irq lock. Thus, after > programming the fb register to fast-track the buffer, the isr also must > be checked to confirm that an interrupt didn't come in the meantime. If > it has, we must assume that programming the register for the > fast-tracked buffer was not in time, and queue the buffer normally. > > Signed-off-by: Paul Elder > Acked-by: Rui Miguel Silva > Reviewed-by: Laurent Pinchart > Signed-off-by: Laurent Pinchart > --- > Changes since v2: > > - Update comments > > Changes since v1: > > - Fix the potential race condition where the interrupt comes in while > the fast tracking routine has the irqlock > - Change return value from int to bool > --- > drivers/staging/media/imx/imx7-media-csi.c | 76 ++++++++++++++++++++++ > 1 file changed, 76 insertions(+) > > diff --git a/drivers/staging/media/imx/imx7-media-csi.c b/drivers/staging/media/imx/imx7-media-csi.c > index a0553c24cce4..ac35cbc16e73 100644 > --- a/drivers/staging/media/imx/imx7-media-csi.c > +++ b/drivers/staging/media/imx/imx7-media-csi.c > @@ -1296,12 +1296,88 @@ static int imx7_csi_video_buf_prepare(struct vb2_buffer *vb) > return 0; > } > > +static bool imx7_csi_fast_track_buffer(struct imx7_csi *csi, > + struct imx7_csi_vb2_buffer *buf) > +{ > + unsigned long flags; > + dma_addr_t phys; > + int buf_num; > + u32 isr; > + > + if (!csi->is_streaming) > + return false; > + > + phys = vb2_dma_contig_plane_dma_addr(&buf->vbuf.vb2_buf, 0); > + > + /* > + * buf_num holds the framebuffer ID of the most recently (*not* the > + * next anticipated) triggered interrupt. Without loss of generality, > + * if buf_num is 0, the hardware is capturing to FB2. If FB1 has been > + * programmed with a dummy buffer (as indicated by active_vb2_buf[0] > + * being NULL), then we can fast-track the new buffer by programming > + * its address in FB1 before the hardware completes FB2, instead of > + * adding it to the buffer queue and incurring a delay of one > + * additional frame. > + * > + * The irqlock prevents races with the interrupt handler that updates > + * buf_num when it programs the next buffer, but we can still race with > + * the hardware if we program the buffer in FB1 just after the hardware > + * completes FB2 and switches to FB1 and before buf_num can be updated > + * by the interrupt handler for FB2. The fast-tracked buffer would > + * then be ignored by the hardware while the driver would think it has > + * successfully been processed. > + * > + * To avoid this problem, if we can't avoid the race, we can detect > + * that we have lost it by checking, after programming the buffer in > + * FB1, if the interrupt flag indicating completion of FB2 has been > + * raised. If that is not the case, fast-tracking succeeded, and we can > + * update active_vb2_buf[0]. Otherwise, we may or may not have lost the > + * race (as the interrupt flag may have been raised just after > + * programming FB1 and before we read the interrupt status register), > + * and we need to assume the worst case of a race loss and queue the > + * buffer through the slow path. > + */ > + > + spin_lock_irqsave(&csi->irqlock, flags); > + > + buf_num = csi->buf_num; > + if (csi->active_vb2_buf[buf_num]) { > + spin_unlock_irqrestore(&csi->irqlock, flags); > + return false; > + } > + > + imx7_csi_update_buf(csi, phys, buf_num); > + > + isr = imx7_csi_reg_read(csi, CSI_CSISR); > + if (isr & (buf_num ? BIT_DMA_TSF_DONE_FB1 : BIT_DMA_TSF_DONE_FB2)) { > + /* > + * The interrupt for the /other/ FB just came (the isr hasn't > + * run yet though, because we have the lock here); we can't be > + * sure we've programmed buf_num FB in time, so queue the buffer > + * to the buffer queue normally. No need to undo writing the FB > + * register, since we won't return it as active_vb2_buf is NULL, > + * so it's okay to potentially write it to both FB1 and FB2; > + * only the one where it was queued normally will be returned. > + */ > + spin_unlock_irqrestore(&csi->irqlock, flags); > + return false; > + } > + > + csi->active_vb2_buf[buf_num] = buf; > + > + spin_unlock_irqrestore(&csi->irqlock, flags); > + return true; > +} > + > static void imx7_csi_video_buf_queue(struct vb2_buffer *vb) > { > struct imx7_csi *csi = vb2_get_drv_priv(vb->vb2_queue); > struct imx7_csi_vb2_buffer *buf = to_imx7_csi_vb2_buffer(vb); > unsigned long flags; > > + if (imx7_csi_fast_track_buffer(csi, buf)) > + return; > + > spin_lock_irqsave(&csi->q_lock, flags); > > list_add_tail(&buf->list, &csi->ready_q); > -- > Regards, > > Laurent Pinchart >