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=-13.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,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 00A9DC433DB for ; Tue, 23 Mar 2021 21:46:14 +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 7F154619CE for ; Tue, 23 Mar 2021 21:46:13 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7F154619CE Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=ideasonboard.com 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 E42C56E921; Tue, 23 Mar 2021 21:46:12 +0000 (UTC) Received: from perceval.ideasonboard.com (perceval.ideasonboard.com [213.167.242.64]) by gabe.freedesktop.org (Postfix) with ESMTPS id 913F26E921 for ; Tue, 23 Mar 2021 21:46:11 +0000 (UTC) Received: from pendragon.ideasonboard.com (62-78-145-57.bb.dnainternet.fi [62.78.145.57]) by perceval.ideasonboard.com (Postfix) with ESMTPSA id B733DB1D; Tue, 23 Mar 2021 22:46:09 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=ideasonboard.com; s=mail; t=1616535970; bh=Uze8l2oHcPC8MtkpUZ2eSJ2rMkiJCYeeUHCf7SRLrrI=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=OUMbWTqT4pd9L4vLfj1WXuDcsp1pG6Ds6vVO66+kOIYPQpPay4vYl78W8uByVnkqv 0X21Y6u58nrJB/wUmnZTDdqhLBcjhBDiENirXfpjSA7undfwAGGW1I2F0JoHjEisqc DSqxaoO1/DbKEtCoKm2KPoag2pknbmvcdRNJubY4= Date: Tue, 23 Mar 2021 23:45:27 +0200 From: Laurent Pinchart To: Doug Anderson Subject: Re: [RFC PATCH 04/11] drm/bridge: ti-sn65dsi86: Use bitmask to store valid rates Message-ID: References: <20210322030128.2283-1-laurent.pinchart+renesas@ideasonboard.com> <20210322030128.2283-5-laurent.pinchart+renesas@ideasonboard.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: 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: Jernej Skrabec , Jonas Karlman , Neil Armstrong , dri-devel , Stephen Boyd , linux-renesas-soc@vger.kernel.org, Andrzej Hajda Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" Hi Doug, On Tue, Mar 23, 2021 at 02:08:55PM -0700, Doug Anderson wrote: > On Sun, Mar 21, 2021 at 8:02 PM Laurent Pinchart wrote: > > > > The valid rates are stored in an array of 8 booleans. Replace it with a > > bitmask to save space. > > I'm curious: do you have evidence that this does anything useful? I > guess you're expecting it to save .text space, right? Stack usage and > execution time differences should be irrelevant--it's not in a > critical section and the difference should be tiny anyway. As far as > .text segment goes, it's not obvious to me that the compiler will use > fewer instructions to manipulate bits compared to booleans. > > Doing a super simple "ls -ah" on vmlinux (unstripped): > > Before: 224820232 bytes > After: 224820376 bytes > > ...so your change made it _bigger_. OK, so running "strip > --strip-debug" on those: > > Before: 26599464 bytes > After: 26599464 bytes > > ...so exactly the same. I tried finding some evidence using "readelf -ah": > > Before: > [ 2] .text PROGBITS ffffffc010010000 00020000 > 0000000000b03508 0000000000000000 WAX 0 0 65536 > [ 3] .rodata PROGBITS ffffffc010b20000 00b30000 > 00000000002e84b3 0000000000000000 WAMS 0 0 4096 > > After: > [ 2] .text PROGBITS ffffffc010010000 00020000 > 0000000000b03508 0000000000000000 WAX 0 0 65536 > [ 3] .rodata PROGBITS ffffffc010b20000 00b30000 > 00000000002e84b3 0000000000000000 WAMS 0 0 4096 > > Maybe you have some evidence showing an improvement? Ah, OK. I > disassembled ti_sn_bridge_enable() and your patch saves 12 bytes, but > I guess maybe alignment washes it out in reality... > > > In terms of readability / conventions, I personally find this change a > bit of a wash. I mean, I guess I originally implemented it as an array > and I thought that was the most readable, but I like bitfields fine > too. If everyone loves it then I won't object, but to me it feels like > touching lines of code for something that's personal preference. ;-) You're right that the .text and CPU time improvements were not my target. I was focussed on stack usage, as that's a limited resource in the kernel. I don't have any evidence that we would be close to any limit, so it's tiny, and if you or anyone else have a strong opinion that an array of booleans is better due to readability concerns, I can drop this change. I only thought about those poor 7 bits in every bool that sat there unused, feeling useless :-) > > Signed-off-by: Laurent Pinchart > > --- > > drivers/gpu/drm/bridge/ti-sn65dsi86.c | 24 +++++++++++++----------- > > 1 file changed, 13 insertions(+), 11 deletions(-) > > > > diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi86.c b/drivers/gpu/drm/bridge/ti-sn65dsi86.c > > index c45420a50e73..1d1be791d5ba 100644 > > --- a/drivers/gpu/drm/bridge/ti-sn65dsi86.c > > +++ b/drivers/gpu/drm/bridge/ti-sn65dsi86.c > > @@ -557,9 +557,9 @@ static int ti_sn_bridge_calc_min_dp_rate_idx(struct ti_sn_bridge *pdata) > > return i; > > } > > > > -static void ti_sn_bridge_read_valid_rates(struct ti_sn_bridge *pdata, > > - bool rate_valid[]) > > +static unsigned int ti_sn_bridge_read_valid_rates(struct ti_sn_bridge *pdata) > > { > > + unsigned int valid_rates = 0; > > unsigned int rate_per_200khz; > > unsigned int rate_mhz; > > u8 dpcd_val; > > @@ -599,13 +599,13 @@ static void ti_sn_bridge_read_valid_rates(struct ti_sn_bridge *pdata, > > j < ARRAY_SIZE(ti_sn_bridge_dp_rate_lut); > > j++) { > > if (ti_sn_bridge_dp_rate_lut[j] == rate_mhz) > > - rate_valid[j] = true; > > + valid_rates |= BIT(j); > > } > > } > > > > for (i = 0; i < ARRAY_SIZE(ti_sn_bridge_dp_rate_lut); i++) { > > - if (rate_valid[i]) > > - return; > > + if (valid_rates & BIT(i)) > > + return valid_rates; > > } > > DRM_DEV_ERROR(pdata->dev, > > "No matching eDP rates in table; falling back\n"); > > @@ -627,15 +627,17 @@ static void ti_sn_bridge_read_valid_rates(struct ti_sn_bridge *pdata, > > (int)dpcd_val); > > fallthrough; > > case DP_LINK_BW_5_4: > > - rate_valid[7] = 1; > > + valid_rates |= BIT(7); > > fallthrough; > > case DP_LINK_BW_2_7: > > - rate_valid[4] = 1; > > + valid_rates |= BIT(4); > > fallthrough; > > case DP_LINK_BW_1_62: > > - rate_valid[1] = 1; > > + valid_rates |= BIT(1); > > break; > > } > > + > > + return valid_rates; > > } > > > > static void ti_sn_bridge_set_video_timings(struct ti_sn_bridge *pdata) > > @@ -753,8 +755,8 @@ static int ti_sn_link_training(struct ti_sn_bridge *pdata, int dp_rate_idx, > > static void ti_sn_bridge_enable(struct drm_bridge *bridge) > > { > > struct ti_sn_bridge *pdata = bridge_to_ti_sn_bridge(bridge); > > - bool rate_valid[ARRAY_SIZE(ti_sn_bridge_dp_rate_lut)] = { }; > > const char *last_err_str = "No supported DP rate"; > > + unsigned int valid_rates; > > int dp_rate_idx; > > unsigned int val; > > int ret = -EINVAL; > > @@ -793,13 +795,13 @@ static void ti_sn_bridge_enable(struct drm_bridge *bridge) > > regmap_update_bits(pdata->regmap, SN_SSC_CONFIG_REG, DP_NUM_LANES_MASK, > > val); > > > > - ti_sn_bridge_read_valid_rates(pdata, rate_valid); > > + valid_rates = ti_sn_bridge_read_valid_rates(pdata); > > > > /* Train until we run out of rates */ > > for (dp_rate_idx = ti_sn_bridge_calc_min_dp_rate_idx(pdata); > > dp_rate_idx < ARRAY_SIZE(ti_sn_bridge_dp_rate_lut); > > dp_rate_idx++) { > > - if (!rate_valid[dp_rate_idx]) > > + if (!(valid_rates & BIT(dp_rate_idx))) > > continue; > > > > ret = ti_sn_link_training(pdata, dp_rate_idx, &last_err_str); > > In any case, since it does save 12 bytes: > > Reviewed-by: Douglas Anderson -- Regards, Laurent Pinchart _______________________________________________ dri-devel mailing list dri-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/dri-devel