linux-kernel - Re: [PATCH] clk-si544: Properly round requested frequency to nearest match

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Message-ID: <152792071704.225090.10103969745756087817@swboyd.mtv.corp.google.com>
Date:   Fri, 01 Jun 2018 23:25:17 -0700
From:   Stephen Boyd <sboyd@...nel.org>
To:     Mike Looijmans <mike.looijmans@...ic.nl>, linux-clk@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, mturquette@...libre.com,
        Mike Looijmans <mike.looijmans@...ic.nl>
Subject: Re: [PATCH] clk-si544: Properly round requested frequency to nearest match

Quoting Mike Looijmans (2018-05-31 07:03:55)
> The si544 driver had a rounding problem that using the result of clk_round_rate
> may set the clock to yet another rate, for example:
> clk_round_rate(195000000) = 194999999
> clk_round_rate(194999999) = 194999998
> 
> Clients would expect that after clk_set_rate(clk, freq2=clk_round_rate(clk, freq)) the
> chip will be running at exactly freq2.
> 
> The problem was in the calculation of the feedback divider, it was always rounded
> down instead of to the nearest possible VCO value.
> 
> After this change, the following holds true for any supported frequency:
> actual_freq = clk_round_rate(clk, freq);
> clk_set_rate(clk, actual_freq);
> clk_round_rate(clk, actual_freq) == actual_freq && clk_get_rate(clk) == actual_freq
> 
> Signed-off-by: Mike Looijmans <mike.looijmans@...ic.nl>
> ---

Applied to clk-next