[PATCH v3] mmc: renesas_sdhi: Fix rounding errors

* [PATCH v3] mmc: renesas_sdhi: Fix rounding errors
@ 2022-09-22  8:55 Biju Das
  2022-09-24 10:18 ` Wolfram Sang
  2022-09-25 16:51 ` Wolfram Sang
  0 siblings, 2 replies; 7+ messages in thread
From: Biju Das @ 2022-09-22  8:55 UTC (permalink / raw)
  To: Ulf Hansson
  Cc: Biju Das, Wolfram Sang, linux-mmc, linux-renesas-soc,
	Geert Uytterhoeven, Chris Paterson, Biju Das,
	Prabhakar Mahadev Lad

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:- (533333333 Hz / 4 * 4 = 533333332 Hz < 533333333 Hz).

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>
---
v2->v3:
 * Renamed the variable new_clock_margin->new_upper_limit in renesas_sdhi_clk_
   update()
 * Moved setting of new_upper_limit outside for loop.
 * Updated the comment section to mention the rounding errors and merged with
   existing comment out side the for loop.
 * Updated commit description. 
v1->v2:
 * Add a comment explaining why margin is needed and set it to
   that particular value.
---
 drivers/mmc/host/renesas_sdhi_core.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/drivers/mmc/host/renesas_sdhi_core.c b/drivers/mmc/host/renesas_sdhi_core.c
index 6edbf5c161ab..45ec15ece1f5 100644
--- 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;
 
 	/*
@@ -153,10 +154,15 @@ 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.
+	 *
+	 * To fix rounding errors, eg:- (533333333 Hz / 4 * 4 = 533333332 Hz <
+	 * 533333333 Hz) add an upper limit of 1/1024 rate higher to the clock
+	 * rate.
 	 */
+	new_upper_limit = (new_clock << i) + ((new_clock << i) >> 10);
 	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)) {
+		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))
@@ -181,6 +187,7 @@ 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 +201,13 @@ 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 */
-- 
2.25.1


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