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=-5.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no 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 6465CC433DB for ; Tue, 2 Feb 2021 15:25:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 28E0F64F54 for ; Tue, 2 Feb 2021 15:25:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235463AbhBBPZf (ORCPT ); Tue, 2 Feb 2021 10:25:35 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39246 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235401AbhBBPYg (ORCPT ); Tue, 2 Feb 2021 10:24:36 -0500 Received: from mail-wm1-x335.google.com (mail-wm1-x335.google.com [IPv6:2a00:1450:4864:20::335]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 01D99C0613ED for ; Tue, 2 Feb 2021 07:23:55 -0800 (PST) Received: by mail-wm1-x335.google.com with SMTP id m2so2850062wmm.1 for ; Tue, 02 Feb 2021 07:23:55 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ffwll.ch; s=google; h=date:from:to:cc:subject:message-id:mail-followup-to:references :mime-version:content-disposition:in-reply-to; bh=PBOv4oa0xyBSu7Av2IkfR/vnH44GMIHIBZqxC79Sm/Y=; b=DS825I+YcxBaWeWEl+ZNWlwgUUIR8iQmQPkY4QFjW+uga7eCjkKj/KLKeVvYlbT342 XZW5+2Ar3ZP7nqC+R9oJZP8ayAuB43dHLEmwnU4GvZ9vZ1Wr26KIXGNKp38uQSy6K9Tt lMA+CYFhvJPF2cC1gm5jEgIe+iEoWYF68Djqs= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id :mail-followup-to:references:mime-version:content-disposition :in-reply-to; bh=PBOv4oa0xyBSu7Av2IkfR/vnH44GMIHIBZqxC79Sm/Y=; b=jWy2T1ESTslk5aLwRR9UeVOFO/MT/nrJI4uZDaURUyGQhmFzh06Xqx7ZBzKxRF46Wz qf71Dg4B5YWVbLDTphyIudnYxVt9KozQXKPBgs7Vw4IGDGOKUcamvON/QwSO8MCRWe/B nsQFZRGU0uBeomW7FiAeOCr9FsRCQe+JEU0A8NgBsJ8v5WyTk+LjtrnzH5vfQfvowX/3 WwhUQl6DcrI3vOYCt8Fof8pDRBsnd2kiidcGfDGuwST9r5NDTmtABQP++PTcGElVBfC3 m8zhA5dQxMxaGOcNZ7GeWD63N1gj1ZzIoM78jnZ1ylgG4mHb+j7snTHHtDUsZXOHpG3y o7Rw== X-Gm-Message-State: AOAM532WGpwwr/2tBoFqfQEz563ltr/+P0QUkRlYX0NBbyO2TnBNaI1f VE3RW1uDF/HcnUBnTM8FHoDMIQ== X-Google-Smtp-Source: ABdhPJwc4Iu2brOpfruPVn+eCfb6X4r3qeK2Ze160BwejVhNYW1tKRv6W4xHtSFBHQEAxujvrMfubQ== X-Received: by 2002:a1c:f417:: with SMTP id z23mr4176491wma.29.1612279433428; Tue, 02 Feb 2021 07:23:53 -0800 (PST) Received: from phenom.ffwll.local ([2a02:168:57f4:0:efd0:b9e5:5ae6:c2fa]) by smtp.gmail.com with ESMTPSA id f6sm3373520wmq.33.2021.02.02.07.23.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 02 Feb 2021 07:23:52 -0800 (PST) Date: Tue, 2 Feb 2021 16:23:50 +0100 From: Daniel Vetter To: Phillip Susi Cc: Daniel Vetter , Geert Uytterhoeven , Linus Torvalds , Pavel Machek , Randy Dunlap , LKML , "linux-doc@vger.kernel.org" , Greg Kroah-Hartman , dri-devel , Linux Fbdev development list Subject: Re: fbcon: remove soft scrollback code (missing Doc. patch) Message-ID: Mail-Followup-To: Phillip Susi , Geert Uytterhoeven , Linus Torvalds , Pavel Machek , Randy Dunlap , LKML , "linux-doc@vger.kernel.org" , Greg Kroah-Hartman , dri-devel , Linux Fbdev development list References: <20200916205434.GA10389@duo.ucw.cz> <87czyf5jjp.fsf@vps.thesusis.net> <87k0s4ai33.fsf@vps.thesusis.net> <87wnvqts9g.fsf@vps.thesusis.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87wnvqts9g.fsf@vps.thesusis.net> X-Operating-System: Linux phenom 5.7.0-1-amd64 Precedence: bulk List-ID: X-Mailing-List: linux-fbdev@vger.kernel.org On Tue, Feb 02, 2021 at 10:13:14AM -0500, Phillip Susi wrote: > > Daniel Vetter writes: > > > Just a quick comment on this: Since most framebuffers are write-combining, > > and reads from that tend to be ~3 orders of magnitude slower than writes > > (at least on the pile of machines I looked at here, there's big > > differences, and some special streaming cpu instructions to make the > > reading side not so slow). > > > > So scrolling by copying tends to be significantly slower than just > > redrawing everything. > > I know this was the case years ago with AGP as iirc, it doubled ( 4x, 8x > ) the PCI clock rate but only for writes wasn't it? I thought this was > no longer an issue with PCIe, but if it is, then I guess I'll go ahead > with cleaning up the dead code and having it re-render with the larger > text buffer. Still the same with PCIe, probably gotten worse since uncached reads are still as slow, but write-combined writes have gotten much faster even. There's work going on to have a coherent link to gpus which would allow fully cached reads and writes, early with nvlink and now as a standard with CXL (https://en.wikipedia.org/wiki/Compute_Express_Link) But that's aimed at big compute jobs for servers, not really for display. Also some on-die gpus have become fully coherent, but again only for render/compute buffers, not for the display framebuffer. So all together 0 signs this is changing going forward, reading from framebuffers is slow. Ok there's some exceptions: For manual update buffers (defio for fbdev drivers, drm also supports this with an entire set of helpers) the framebuffer used by the cpu is sometimes (but very often still not) cached. Imo not worth optimizing for, since for the drivers where it is cached they either have no blitter, or it's really tiny panels behind spi links and similar, so not going to be fast anyway. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch