mmc: renesas_sdhi: Fix rounding errors

Due to clk rounding errors on RZ/G2L platforms, it selects a clock source
with a lower clock rate compared to a higher one.
For eg: The rounding error (533333333 Hz / 4 * 4 = 533333332 Hz < 5333333
33 Hz) selects a clk source of 400 MHz instead of 533.333333 MHz.

This patch fixes this issue by adding a margin of (1/1024) higher to
the clock rate.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Fixes: bb6d3fa98a41 ("clk: renesas: rcar-gen3: Switch to new SD clock handling")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20220928110755.849275-1-biju.das.jz@bp.renesas.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

diff --git a/drivers/mmc/host/renesas_sdhi_core.c b/drivers/mmc/host/renesas_sdhi_core.c
index 6edbf5c..b970699 100644 (file)
--- a/drivers/mmc/host/renesas_sdhi_core.c
+++ b/drivers/mmc/host/renesas_sdhi_core.c
@@ -128,6 +128,7 @@ static unsigned int renesas_sdhi_clk_update(struct tmio_mmc_host *host,
        struct clk *ref_clk = priv->clk;
        unsigned int freq, diff, best_freq = 0, diff_min = ~0;
        unsigned int new_clock, clkh_shift = 0;
+       unsigned int new_upper_limit;
        int i;
 
        /*
@@ -153,13 +154,20 @@ static unsigned int renesas_sdhi_clk_update(struct tmio_mmc_host *host,
         * greater than, new_clock.  As we can divide by 1 << i for
         * any i in [0, 9] we want the input clock to be as close as
         * possible, but no greater than, new_clock << i.
+        *
+        * Add an upper limit of 1/1024 rate higher to the clock rate to fix
+        * clk rate jumping to lower rate due to rounding error (eg: RZ/G2L has
+        * 3 clk sources 533.333333 MHz, 400 MHz and 266.666666 MHz. The request
+        * for 533.333333 MHz will selects a slower 400 MHz due to rounding
+        * error (533333333 Hz / 4 * 4 = 533333332 Hz < 533333333 Hz)).
         */
        for (i = min(9, ilog2(UINT_MAX / new_clock)); i >= 0; i--) {
                freq = clk_round_rate(ref_clk, new_clock << i);
-               if (freq > (new_clock << i)) {
+               new_upper_limit = (new_clock << i) + ((new_clock << i) >> 10);
+               if (freq > new_upper_limit) {
                        /* Too fast; look for a slightly slower option */
                        freq = clk_round_rate(ref_clk, (new_clock << i) / 4 * 3);
-                       if (freq > (new_clock << i))
+                       if (freq > new_upper_limit)
                                continue;
                }
 
@@ -181,6 +189,7 @@ static unsigned int renesas_sdhi_clk_update(struct tmio_mmc_host *host,
 static void renesas_sdhi_set_clock(struct tmio_mmc_host *host,
                                   unsigned int new_clock)
 {
+       unsigned int clk_margin;
        u32 clk = 0, clock;
 
        sd_ctrl_write16(host, CTL_SD_CARD_CLK_CTL, ~CLK_CTL_SCLKEN &
@@ -194,7 +203,13 @@ static void renesas_sdhi_set_clock(struct tmio_mmc_host *host,
        host->mmc->actual_clock = renesas_sdhi_clk_update(host, new_clock);
        clock = host->mmc->actual_clock / 512;
 
-       for (clk = 0x80000080; new_clock >= (clock << 1); clk >>= 1)
+       /*
+        * Add a margin of 1/1024 rate higher to the clock rate in order
+        * to avoid clk variable setting a value of 0 due to the margin
+        * provided for actual_clock in renesas_sdhi_clk_update().
+        */
+       clk_margin = new_clock >> 10;
+       for (clk = 0x80000080; new_clock + clk_margin >= (clock << 1); clk >>= 1)
                clock <<= 1;
 
        /* 1/1 clock is option */