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=-9.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,USER_AGENT_NEOMUTT 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 3519AC04EBF for ; Tue, 4 Dec 2018 10:26:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E3B43214D9 for ; Tue, 4 Dec 2018 10:26:43 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E3B43214D9 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=mess.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726099AbeLDK0n (ORCPT ); Tue, 4 Dec 2018 05:26:43 -0500 Received: from gofer.mess.org ([88.97.38.141]:52579 "EHLO gofer.mess.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725767AbeLDK0m (ORCPT ); Tue, 4 Dec 2018 05:26:42 -0500 Received: by gofer.mess.org (Postfix, from userid 1000) id 6C51360B84; Tue, 4 Dec 2018 10:26:40 +0000 (GMT) Date: Tue, 4 Dec 2018 10:26:40 +0000 From: Sean Young To: Mauro Carvalho Chehab Cc: Nick Desaulniers , Nathan Chancellor , Mauro Carvalho Chehab , linux-media@vger.kernel.org, LKML Subject: Re: [PATCH] [media] dib7000p: Remove dead code Message-ID: <20181204102639.3qsvfxrzmsvybiop@gofer.mess.org> References: <20180915054739.14117-1-natechancellor@gmail.com> <20180917193936.33e90d5a@coco.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180917193936.33e90d5a@coco.lan> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Sep 17, 2018 at 07:39:36PM -0300, Mauro Carvalho Chehab wrote: > Em Mon, 17 Sep 2018 10:58:32 -0700 > Nick Desaulniers escreveu: > > > On Fri, Sep 14, 2018 at 10:47 PM Nathan Chancellor > > wrote: > > > > > > Clang warns that 'interleaving' is assigned to itself in this function. > > > > > > drivers/media/dvb-frontends/dib7000p.c:1874:15: warning: explicitly > > > assigning value of variable of type 'int' to itself [-Wself-assign] > > > interleaving = interleaving; > > > ~~~~~~~~~~~~ ^ ~~~~~~~~~~~~ > > > 1 warning generated. > > > > > > It's correct. Just removing the self-assignment would sufficiently hide > > > the warning but all of this code is dead because 'tmp' is zero due to > > > being multiplied by zero. This doesn't appear to be an issue with > > > dib8000, which this code was copied from in commit 041ad449683b > > > ("[media] dib7000p: Add DVBv5 stats support"). > > > > > > Reported-by: Nick Desaulniers > > > Signed-off-by: Nathan Chancellor > > > --- > > > drivers/media/dvb-frontends/dib7000p.c | 10 ++-------- > > > 1 file changed, 2 insertions(+), 8 deletions(-) > > > > > > diff --git a/drivers/media/dvb-frontends/dib7000p.c b/drivers/media/dvb-frontends/dib7000p.c > > > index 58387860b62d..25843658fc68 100644 > > > --- a/drivers/media/dvb-frontends/dib7000p.c > > > +++ b/drivers/media/dvb-frontends/dib7000p.c > > > @@ -1800,9 +1800,8 @@ static u32 dib7000p_get_time_us(struct dvb_frontend *demod) > > > > Something looks wrong here (with this function). The patch is no > > functional change, since as you point out `interleaving` is > > initialized to 0, then never updated before read, but I think there's > > an underlying bug here that should be fixed differently. Thanks for > > the patch though, as it does raise the question. > > > > dib7000p_get_time_us has this comment above it: > > > > 1798 /* FIXME: may require changes - this one was borrowed from > > dib8000 */ > > The goal of dib7000p_get_time_us() is to estimate how much time it > takes, with current tuning parameters, to have a certain number of > DVB-T packets. This is used for block error count. That's said, > on a quick look, it seems that the code is not right on many ways. > > It should be aligned with the amount of data it is required for > dib7000 to update the block/bit error counters. There are two kinds > of implementation: > > 1) the frontend has an internal counter that it is shifted and made > available to the driver after a certain amount of received data > (usually in the order of 10^5 to 10^7 bits); > > 2) the frontend has an internal timer that shifts the data from its > internal counter after a certain amount of time (usually at the > seconds range). > > Different vendors opt for either one of the strategy. Some updates > a counter with the amount of bits taken. Unfortunately, this is not > the case of those dib* frontends. So, the Kernel has to estimate > it, based on the tuning parameters. > > From the code, it seems that, for block errors, it waits for 1,250,000 > bits to arrive (e. g. about 766 packets), so, it uses type (1) strategy: > > /* Estimate the number of packets based on bitrate */ > if (!time_us) > time_us = dib7000p_get_time_us(demod); > > if (time_us) { > blocks = 1250000ULL * 1000000ULL; // the multiply here is to convert to microsseconds... > do_div(blocks, time_us * 8 * 204); // As here it divides by the time in microsseconds > c->block_count.stat[0].scale = FE_SCALE_COUNTER; > c->block_count.stat[0].uvalue += blocks; > } > > For BER, the logic assumes that the bit error count should be divided > by 10^-8: > > c->post_bit_count.stat[0].uvalue += 100000000; > > and the counter is updated every second. So, it uses (2). > > > > > Wondering if it has the same bug, it seems it does not: > > drivers/media/dvb-frontends/dib8000.c#dib8000_get_time_us():3986 > > > > dib8000_get_time_us() seems to loop over multiple layers, and then > > assigns interleaving to the final layers interleaving (that looks like > > loop invariant code to me). > > > > Mauro, should dib7000p_get_time_us() use c->layer[???].interleaving? > > I don't think that time interleaving would affect the bit rate. > I suspect that the dead code on dib8000 is just a dead code. > > > I don't see a single reference to `layer` in > > drivers/media/dvb-frontends/dib7000p.c. > > Layers are specific for ISDB-T, but I think DVB-T (or at least DVB-T2) > may use time interleaving. > > Yet, as I said, the goal is to estimate the streaming bit rate. > > I don't remember anymore from where the dib8000 formula came. > > My guts tell that time interleaving shouldn't do much changes (if any) > to the bit rate. I suspect that removing the dead code is likely > OK, but I'll try to see if I can find something related to where this > formula came. So we have two issues. One is the clang issue and clearly the code needs fixing up. The second issue is that we're not sure about the algorithm; I've been reading up on mpeg-ts but I'm not anywhere near getting to an answer on this. How about we merge a patch which just fixes the clang issue and leave the rest of the code as-is for now? Thanks, Sean --- >From c6e4c5f514c38511d2054c69f7b103e98c520af4 Mon Sep 17 00:00:00 2001 From: Sean Young Date: Tue, 4 Dec 2018 09:59:10 +0000 Subject: [PATCH v2] media: dib7000p: Remove dead code Clang warns that 'interleaving' is assigned to itself in this function. drivers/media/dvb-frontends/dib7000p.c:1874:15: warning: explicitly assigning value of variable of type 'int' to itself [-Wself-assign] interleaving = interleaving; ~~~~~~~~~~~~ ^ ~~~~~~~~~~~~ 1 warning generated. Just remove the self-assign and leave existing code in place for now. Reported-by: Nick Desaulniers Signed-off-by: Sean Young --- drivers/media/dvb-frontends/dib7000p.c | 2 -- 1 file changed, 2 deletions(-) diff --git a/drivers/media/dvb-frontends/dib7000p.c b/drivers/media/dvb-frontends/dib7000p.c index 58387860b62d..cd84320c61c9 100644 --- a/drivers/media/dvb-frontends/dib7000p.c +++ b/drivers/media/dvb-frontends/dib7000p.c @@ -1871,8 +1871,6 @@ static u32 dib7000p_get_time_us(struct dvb_frontend *demod) break; } - interleaving = interleaving; - denom = bits_per_symbol * rate_num * fft_div * 384; /* If calculus gets wrong, wait for 1s for the next stats */ -- 2.19.2