mmc: cb710: partially demystify clock selection

Clock divider selection in partially verified, so document known facts
in code.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Chris Ball <cjb@laptop.org>

1 files changed, 12 insertions, 7 deletions

diff --git a/drivers/mmc/host/cb710-mmc.c b/drivers/mmc/host/cb710-mmc.c
index ca3bdc831900..ad61c4c11071 100644
--- a/drivers/mmc/host/cb710-mmc.c
+++ b/drivers/mmc/host/cb710-mmc.c
@@ -25,7 +25,7 @@ static const u8 cb710_src_freq_mhz[16] = {
         50, 55, 60, 65, 70, 75, 80, 85
 };
 
-static void cb710_mmc_set_clock(struct mmc_host *mmc, int hz)
+static void cb710_mmc_select_clock_divider(struct mmc_host *mmc, int hz)
 {
         struct cb710_slot *slot = cb710_mmc_to_slot(mmc);
         struct pci_dev *pdev = cb710_slot_to_chip(slot)->pdev;
@@ -33,8 +33,11 @@ static void cb710_mmc_set_clock(struct mmc_host *mmc, int hz)
         u32 divider_idx;
         int src_hz;
 
-        /* this is magic, unverifiable for me, unless I get
-         * MMC card with cables connected to bus signals */
+        /* on CB710 in HP nx9500:
+         *   src_freq_idx == 0
+         *   indexes 1-7 work as written in the table
+         *   indexes 0,8-15 give no clock output
+         */
         pci_read_config_dword(pdev, 0x48, &src_freq_idx);
         src_freq_idx = (src_freq_idx >> 16) & 0xF;
         src_hz = cb710_src_freq_mhz[src_freq_idx] * 1000000;
@@ -46,13 +49,15 @@ static void cb710_mmc_set_clock(struct mmc_host *mmc, int hz)
 
         if (src_freq_idx)
                 divider_idx |= 0x8;
+        else if (divider_idx == 0)
+                divider_idx = 1;
 
         cb710_pci_update_config_reg(pdev, 0x40, ~0xF0000000, divider_idx << 28);
 
         dev_dbg(cb710_slot_dev(slot),
-                "clock set to %d Hz, wanted %d Hz; flag = %d\n",
+                "clock set to %d Hz, wanted %d Hz; src_freq_idx = %d, divider_idx = %d|%d\n",
                 src_hz >> cb710_clock_divider_log2[divider_idx & 7],
-                hz, (divider_idx & 8) != 0);
+                hz, src_freq_idx, divider_idx & 7, divider_idx & 8);
 }
 
 static void __cb710_mmc_enable_irq(struct cb710_slot *slot,
@@ -512,7 +517,7 @@ static int cb710_mmc_powerup(struct cb710_slot *slot)
 #endif
         int err;
 
-        /* a lot of magic; see comment in cb710_mmc_set_clock() */
+        /* a lot of magic for now */
         dev_dbg(cb710_slot_dev(slot), "bus powerup\n");
         cb710_dump_regs(chip, CB710_DUMP_REGS_MMC);
         err = cb710_wait_while_busy(slot, CB710_MMC_S2_BUSY_20);
@@ -572,7 +577,7 @@ static void cb710_mmc_set_ios(struct mmc_host *mmc, struct mmc_ios *ios)
         struct cb710_mmc_reader *reader = mmc_priv(mmc);
         int err;
 
-        cb710_mmc_set_clock(mmc, ios->clock);
+        cb710_mmc_select_clock_divider(mmc, ios->clock);
 
         if (!cb710_mmc_is_card_inserted(slot)) {
                 dev_dbg(cb710_slot_dev(slot),