[PATCH v2 1/9] media: i2c: ov7251: Use in-range PLL1 dividers for the 319.2 MHz link

[Antoine Pichot]

The 19.2 MHz PLL1 table for the 319.2 MHz link frequency puts a
pre-divider of 1 before the multiplier of 133 and a divider of 4 after
it, so the PLL1 multiplier output (the VCO) runs at 19.2 x 133 =
2553.6 MHz before being divided down to 638.4 MHz. The OV7750/OV7251
datasheet (rev 2.12, table 2-11) limits it to 300 to 800 MHz.

Use pre-divider 4 (register 0x30B4 = 0x04) and divider 1 (0x30B1 =
0x01). The multiplier output is then 4.8 x 133 = 638.4 MHz, inside the
range, and the MIPI clock stays 638.4 Mbps, so the link frequency does
not change. The Windows driver programs these values for the Surface
Book 2's Windows Hello mode.

On ACPI systems this table has been unreachable since it was added,
because ov7251_check_hwcfg() selects the wrong index. The next patch
fixes the index, so the two must go in together. With the index fix
alone, a Surface Pro 7+ (IPU6) had CSI-2 errors in 7 of 10 150-frame
runs, 3 of them stalls; with this change 0 of 13. A Surface Book 2
(IPU3) streams clean with either table. Dave Stevenson measured on a
24 MHz Raspberry Pi board that the out-of-range tables miss their
nominal frequency and proposed the same values. The 24 MHz tables are
left alone because nobody in the thread can test a change to them.

Fixes: ed9566ce1946 ("media: i2c: Add support for new frequencies to ov7251")
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lore.kernel.org/linux-media/6ae0e944.b6db0abd.16b13d.d261@xxxxxxxxxxxxx/
Link: https://lore.kernel.org/linux-media/CAPY8ntBFJJPg3y46gC=bNq_55QifC7mKmeq8+pAEDVN42fGUXw@xxxxxxxxxxxxxx/
Signed-off-by: Antoine Pichot <antoine.pichot@xxxxxxxxx>
---
 drivers/media/i2c/ov7251.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/media/i2c/ov7251.c b/drivers/media/i2c/ov7251.c
index fce6344b5120..f429de5f1f7c 100644
--- a/drivers/media/i2c/ov7251.c
+++ b/drivers/media/i2c/ov7251.c
@@ -174,9 +174,9 @@ static const struct ov7251_pll1_cfg ov7251_pll1_cfg_19_2_mhz_240_mhz = {
 };
 
 static const struct ov7251_pll1_cfg ov7251_pll1_cfg_19_2_mhz_319_2_mhz = {
-	.pre_div = 0x01,
+	.pre_div = 0x04,
 	.mult = 0x85,
-	.div = 0x04,
+	.div = 0x01,
 	.pix_div = 0x0a,
 	.mipi_div = 0x05,
 };
-- 
2.56.0