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 X-Spam-Level: X-Spam-Status: No, score=-8.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 849C5C49EA5 for ; Thu, 24 Jun 2021 12:01:17 +0000 (UTC) Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 2980D613DC for ; Thu, 24 Jun 2021 12:01:17 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2980D613DC Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=pengutronix.de Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=dri-devel-bounces@lists.freedesktop.org Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id A8A6D6EB5C; Thu, 24 Jun 2021 12:01:16 +0000 (UTC) Received: from metis.ext.pengutronix.de (metis.ext.pengutronix.de [IPv6:2001:67c:670:201:290:27ff:fe1d:cc33]) by gabe.freedesktop.org (Postfix) with ESMTPS id 002886EB5C for ; Thu, 24 Jun 2021 12:01:15 +0000 (UTC) Received: from gallifrey.ext.pengutronix.de ([2001:67c:670:201:5054:ff:fe8d:eefb] helo=[IPv6:::1]) by metis.ext.pengutronix.de with esmtps (TLS1.3:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1lwO2k-0001GB-Jf; Thu, 24 Jun 2021 14:01:14 +0200 Message-ID: Subject: Re: [PATCH] drm: mxsfb: Clear FIFO_CLEAR bit From: Lucas Stach To: Marek Vasut , dri-devel@lists.freedesktop.org Date: Thu, 24 Jun 2021 14:01:13 +0200 In-Reply-To: References: <20210620224946.189524-1-marex@denx.de> <85372867-2b5b-e97f-aa04-ed976db1eddb@denx.de> <07f58c1181e2e66277d0355055ff794fb091991d.camel@pengutronix.de> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.40.1 (3.40.1-1.fc34) MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-SA-Exim-Connect-IP: 2001:67c:670:201:5054:ff:fe8d:eefb X-SA-Exim-Mail-From: l.stach@pengutronix.de X-SA-Exim-Scanned: No (on metis.ext.pengutronix.de); SAEximRunCond expanded to false X-PTX-Original-Recipient: dri-devel@lists.freedesktop.org X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Daniel Abrecht , Emil Velikov , ch@denx.de, Laurent Pinchart Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" Am Dienstag, dem 22.06.2021 um 11:33 +0200 schrieb Marek Vasut: > On 6/22/21 9:28 AM, Lucas Stach wrote: > > Am Montag, dem 21.06.2021 um 18:30 +0200 schrieb Marek Vasut: > > > On 6/21/21 2:14 PM, Lucas Stach wrote: > > > > > > [...] > > > > > > > > diff --git a/drivers/gpu/drm/mxsfb/mxsfb_kms.c b/drivers/gpu/drm/mxsfb/mxsfb_kms.c > > > > > index 98d8ba0bae84..22cb749fc9bc 100644 > > > > > --- a/drivers/gpu/drm/mxsfb/mxsfb_kms.c > > > > > +++ b/drivers/gpu/drm/mxsfb/mxsfb_kms.c > > > > > @@ -241,6 +241,9 @@ static void mxsfb_crtc_mode_set_nofb(struct mxsfb_drm_private *mxsfb, > > > > > > > > > > /* Clear the FIFOs */ > > > > > writel(CTRL1_FIFO_CLEAR, mxsfb->base + LCDC_CTRL1 + REG_SET); > > > > > + readl(mxsfb->base + LCDC_CTRL1); > > > > > > > > Do you really need those readbacks? As both writes are targeting the > > > > same slave interface, the memory barrier in the clear write should push > > > > the set write. > > > > > > What would push the clear write then ? We can drop one of the readl()s, > > > but not the last one. > > > > There are a lot of more writes with barriers to the controller slave > > interface in that function after clearing the FIFO. I don't see why > > this readback would be required. > > Because you really do want to make sure the fifo is cleared before you > start doing any of those other writes or configuring the controller in > any way. I still don't see the reason. What additional properties do you think the readback provides that isn't already provided by the barriers in the following writes? Regards, Lucas