From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751165AbdAWNHf (ORCPT ); Mon, 23 Jan 2017 08:07:35 -0500 Received: from dougal.metanate.com ([90.155.101.14]:38433 "EHLO metanate.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1750723AbdAWNHe (ORCPT ); Mon, 23 Jan 2017 08:07:34 -0500 Date: Mon, 23 Jan 2017 12:49:25 +0000 From: John Keeping To: Chris Zhong Cc: Mark Yao , dri-devel@lists.freedesktop.org, linux-arm-kernel@lists.infradead.org, linux-rockchip@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 19/26] drm/rockchip: dw-mipi-dsi: improve PLL configuration Message-ID: <20170123124925.4fa10fc2.john@metanate.com> In-Reply-To: <58855EAE.2010704@rock-chips.com> References: <20170121163128.22240-1-john@metanate.com> <20170121163128.22240-20-john@metanate.com> <58855EAE.2010704@rock-chips.com> Organization: Metanate Ltd X-Mailer: Claws Mail 3.14.1 (GTK+ 2.24.31; x86_64-unknown-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Chris, On Mon, 23 Jan 2017 09:38:54 +0800, Chris Zhong wrote: > On 01/22/2017 12:31 AM, John Keeping wrote: > > The multiplication ratio for the PLL is required to be even due to the > > use of a "by 2 pre-scaler". Currently we are likely to end up with an > > odd multiplier even though there is an equivalent set of parameters with > > an even multiplier. > > > > For example, using the 324MHz bit rate with a reference clock of 24MHz > > we end up with M = 27, N = 2 whereas the example in the PHY databook > > gives M = 54, N = 4 for this bit rate and reference clock. > > > > By walking down through the available multiplier instead of up we are > > more likely to hit an even multiplier. With the above example we do now > > get M = 54, N = 4 as given by the databook. > > > > While doing this, change the loop limits to encode the actual limits on > > the divisor, which are: > > > > 40MHz >= (pllref / N) >= 5MHz > > This formula is limit for N, but we still can not guarantee to get an > even M. > Do you think we should do a check for M. > such as: > if (m % 2) > continue; > ... > for (i = pllref / 5; i > (pllref / 40); i--) { > pre = pllref / i; > if ((tmp > (target_mbps % pre)) && (target_mbps / pre < 512)) { > tmp = target_mbps % pre; > n = i; > m = target_mbps / pre; > if (m % 2) > continue; > } > if (tmp == 0) > break; > } > > if (m % 2) > m++; > > dsi->lane_mbps = pllref / n * m; > dsi->input_div = n; > dsi->feedback_div = m; Yes, I agree that there should be a check for M, but I'm not sure if the version above is sufficient. The "m % 2" check inside the loop means that we don't break immediately when tmp=0 but then we are guaranteed to break next time without having modified n, m because now tmp=0 so "tmp > (target_mbps % pre)" is always false and we just hit the "if (tmp == 0) break" case next time. Given that the descending loop already means that if we can hit "tmp" exactly we are more likely to do so with a bigger N and even M, I think it might be better to just fix M after the loop like: if (m % 2) { if (m < 256 && (n * 2) <= (pllref / 5)) { n *= 2; m *= 2; } else { m++; } } but I haven't thought about this too carefully. For this series, I'd rather either keep this patch as it is or drop it in favour of a more comprehensive solution. I don't want to block the other fixes waiting for a perfect fix here and we can always improve this further with a follow-up patch. > > Signed-off-by: John Keeping > > --- > > Unchanged in v2 > > --- > > drivers/gpu/drm/rockchip/dw-mipi-dsi.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/drivers/gpu/drm/rockchip/dw-mipi-dsi.c b/drivers/gpu/drm/rockchip/dw-mipi-dsi.c > > index 12432e41971b..f2320cf1366c 100644 > > --- a/drivers/gpu/drm/rockchip/dw-mipi-dsi.c > > +++ b/drivers/gpu/drm/rockchip/dw-mipi-dsi.c > > @@ -519,7 +519,7 @@ static int dw_mipi_dsi_get_lane_bps(struct dw_mipi_dsi *dsi, > > pllref = DIV_ROUND_UP(clk_get_rate(dsi->pllref_clk), USEC_PER_SEC); > > tmp = pllref; > > > > - for (i = 1; i < 6; i++) { > > + for (i = pllref / 5; i > (pllref / 40); i--) { > > pre = pllref / i; > > if ((tmp > (target_mbps % pre)) && (target_mbps / pre < 512)) { > > tmp = target_mbps % pre; > >