Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756908AbbLBWVY (ORCPT ); Wed, 2 Dec 2015 17:21:24 -0500 Received: from relay2-d.mail.gandi.net ([217.70.183.194]:42747 "EHLO relay2-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753471AbbLBWVW (ORCPT ); Wed, 2 Dec 2015 17:21:22 -0500 X-Originating-IP: 81.57.43.44 Date: Wed, 2 Dec 2015 23:21:52 +0100 From: Remi Pommarel To: Eric Anholt Cc: Stephen Warren , Lee Jones , Michael Turquette , Stephen Boyd , linux-rpi-kernel@lists.infradead.org, linux-clk@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 1/3] clk: bcm2835: Always round up clock divisor Message-ID: <20151202222152.GH12775@cruxbox> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151118191141.GB491@cruxbox> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2866 Lines: 74 Hi Eric On Wed, Nov 18, 2015 at 07:11:41PM -0100, Remi Pommarel wrote: > On Wed, Nov 18, 2015 at 10:25:45AM -0800, Eric Anholt wrote: > > Remi Pommarel writes: > > > > > Make bcm2835_clock_choose_div always round up the chosen MASH divisor so that > > > the resulting average rate will not be higher than the requested one. > > > > > > Signed-off-by: Remi Pommarel > > > --- > > > drivers/clk/bcm/clk-bcm2835.c | 15 ++++++++------- > > > 1 file changed, 8 insertions(+), 7 deletions(-) > > > > > > diff --git a/drivers/clk/bcm/clk-bcm2835.c b/drivers/clk/bcm/clk-bcm2835.c > > > index 39bf582..1237716 100644 > > > --- a/drivers/clk/bcm/clk-bcm2835.c > > > +++ b/drivers/clk/bcm/clk-bcm2835.c > > > @@ -1152,18 +1152,19 @@ static u32 bcm2835_clock_choose_div(struct clk_hw *hw, > > > { > > > struct bcm2835_clock *clock = bcm2835_clock_from_hw(hw); > > > const struct bcm2835_clock_data *data = clock->data; > > > - u32 unused_frac_mask = GENMASK(CM_DIV_FRAC_BITS - data->frac_bits, 0); > > > + u32 unused_frac_mask = > > > + GENMASK(CM_DIV_FRAC_BITS - data->frac_bits, 0) >> 1; > > > u64 temp = (u64)parent_rate << CM_DIV_FRAC_BITS; > > > + u64 rem; > > > u32 div; > > > > > > - do_div(temp, rate); > > > + rem = do_div(temp, rate); > > > div = temp; > > > > > > - /* Round and mask off the unused bits */ > > > - if (unused_frac_mask != 0) { > > > - div += unused_frac_mask >> 1; > > > - div &= ~unused_frac_mask; > > > - } > > > + /* Round up and mask off the unused bits */ > > > + if ((div & unused_frac_mask) != 0 || rem != 0) > > > + div += unused_frac_mask + 1; > > > + div &= ~unused_frac_mask; > > > > Suppose we've got 8 of our 12 frac bits populated. You've added a ">> > > 1" to the unused_frac_mask, so it's only 0x7 instead of 0xf. When you > > say "round up", you add 0x8 (the high bit of the unused mask") then and > > with ~0x7. If you started with 0x1 in the low bits of div, you'd end up > > with 0x8, so you've set an unused bit instead of actually rounding up. > > > > Did my logic work, here? I think you just want to drop the ">>1" in > > unused_frac_mask. > > I don't think so. > > If we have 8 of our 12 frac bits populated GENMASK(12 - 8, 0) will be 0x1f > because GENMASK(4, 0) generates a mask from bit at position 0 to bit at > position 4 inclusively (which is the fifth bit). So GENMASK(4, 0) >> 1 will > be 0xf which is what we want here. Does my logic seems sensible to you or am I missing something ? I just want to be sure we agree before I send another version for this patchset. Thanks. Best Regards -- Remi -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/