Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754714AbbK3PgO (ORCPT ); Mon, 30 Nov 2015 10:36:14 -0500 Received: from mail-wm0-f42.google.com ([74.125.82.42]:38115 "EHLO mail-wm0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754061AbbK3PgL (ORCPT ); Mon, 30 Nov 2015 10:36:11 -0500 MIME-Version: 1.0 In-Reply-To: <1448883819.15961.7.camel@mtksdaap41> References: <1447764885-23100-1-git-send-email-tiffany.lin@mediatek.com> <1447764885-23100-4-git-send-email-tiffany.lin@mediatek.com> <5655DDB4.2080002@linaro.org> <1448626209.7734.26.camel@mtksdaap41> <56584AC5.7020704@linaro.org> <1448883819.15961.7.camel@mtksdaap41> Date: Mon, 30 Nov 2015 15:36:07 +0000 Message-ID: Subject: Re: [RESEND RFC/PATCH 3/8] media: platform: mtk-vpu: Support Mediatek VPU From: Daniel Thompson To: andrew-ct chen Cc: Mark Rutland , James Liao , Catalin Marinas , Will Deacon , Darren Etheridge , Laurent Pinchart , Eddie Huang , Pawel Moll , Hongzhou Yang , Mauro Carvalho Chehab , Fabien Dessenne , Peter Griffin , Geert Uytterhoeven , Mikhail Ulyanov , Hans Verkuil , linux-media@vger.kernel.org, "devicetree@vger.kernel.org" , Arnd Bergmann , Ian Campbell , Sascha Hauer , Benoit Parrot , Rob Herring , linux-mediatek@lists.infradead.org, Yingjoe Chen , Matthias Brugger , Tiffany Lin , "linux-arm-kernel@lists.infradead.org" , Daniel Hsiao , lkml , Sakari Ailus , Kumar Gala Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3139 Lines: 77 On 30 November 2015 at 11:43, andrew-ct chen wrote: > On Fri, 2015-11-27 at 12:21 +0000, Daniel Thompson wrote: >> On 27/11/15 12:10, andrew-ct chen wrote: >> >>> + >> >>> > >+ memcpy((void *)send_obj->share_buf, buf, len); >> >>> > >+ send_obj->len = len; >> >>> > >+ send_obj->id = id; >> >>> > >+ vpu_cfg_writel(vpu, 0x1, HOST_TO_VPU); >> >>> > >+ >> >>> > >+ /* Wait until VPU receives the command */ >> >>> > >+ timeout = jiffies + msecs_to_jiffies(IPI_TIMEOUT_MS); >> >>> > >+ do { >> >>> > >+ if (time_after(jiffies, timeout)) { >> >>> > >+ dev_err(vpu->dev, "vpu_ipi_send: IPI timeout!\n"); >> >>> > >+ return -EIO; >> >>> > >+ } >> >>> > >+ } while (vpu_cfg_readl(vpu, HOST_TO_VPU)); >> >> > >> >> >Do we need to busy wait every time we communicate with the co-processor? >> >> >Couldn't we put this wait*before* we write to HOST_TO_VPU above. >> >> > >> >> >That way we only spin when there is a need to. >> >> > >> > Since the hardware VPU only allows that one client sends the command to >> > it each time. >> > We need the wait to make sure VPU accepted the command and cleared the >> > interrupt and then the next command would be served. >> >> I understand that the VPU can only have on message outstanding at once. >> >> I just wonder why we busy wait *after* sending the first command rather >> than *before* sending the second one. > > No other special reasons. Just send one command and wait until VPU gets > the command. Then, I think this wait also can be put before we write to > HOST_TO_VPU.Is this better than former? May I know the reason? Busy waiting is bad; it is a waste of host CPU processor time and/or power. When the busy wait occurs after queuing then we will busy wait for every command we send. If busy wait occurs before next queuing then we will wait for a shorter time in total because we have the chance to do something useful on the host before we have to wait. >> Streamed decode/encode typically ends up being rate controlled by >> capture or display meaning that in these cases we don't need to busy >> wait at all (because by the time we send the next frame the VPU has >> already accepted the previous message). > > For now, only one device "encoder" exists, it is true. > But, we'll have encoder and decoder devices, the decode and encode > requested to VPU are simultaneous. Sure, I accept that lock and busy-wait is an appropriate way to ensure safety (assuming the VPU is fairly quick clearing the HOST_TO_VPU flag). > Is this supposed to be removed for this patches and we can add it back > if the another device(decoder) is ready for review? No I'm not suggesting that. I only recommend moving the busy wait *before* end sending of command (for efficiency reasons mentioned above). Daniel. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/