Re: [PATCH 1/1] mmc: sdhci-bcm2835: added quirk and removed udelay in write ops
From: Scott Branden
Date: Sat Oct 18 2014 - 02:41:07 EST
Great review - thanks.
On 14-10-17 07:37 PM, Stephen Warren wrote:
On 10/15/2014 10:43 AM, Scott Branden wrote:
Added quirk SDHCI_QUIRK_MULTIBLOCK_READ_ACMD12 present in controller.
Removed udelay in write ops by using shadow registers for 16 bit
accesses to 32-bit registers (where necessary).
Optimized 32-bit operations when doing 8/16 register accesses.
That's 2 or 3 unrelated changes. They'd be better as separate patches,
so that any issues that arise can be bisected down to the smaller changes.
OK - I will split into smaller patches to bisect and understand better.
diff --git a/drivers/mmc/host/sdhci-bcm2835.c b/drivers/mmc/host/sdhci-bcm2835.c
/*
* The Arasan has a bugette whereby it may lose the content of successive
+ * writes to the same register that are within two SD-card clock cycles of
+ * each other (a clock domain crossing problem). Problem does not happen with
^ The?
See right >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> ^
+ * data.
Blank line to separate the paragraphs here, to be consistent with the
other paragraph break below?
I'll clean up the comment some more.
+ * This wouldn't be a problem with the code except that we can only write the
+ * controller with 32-bit writes. So two different 16-bit registers in the
+ * written back to back creates the problem.
*
+ * In reality, this only happens when a SDHCI_BLOCK_SIZE and SDHCI_BLOCK_COUNT
+ * are written followed by SDHCI_TRANSFER_MODE and SDHCI_COMMAND.
That seems like a rather risky assertion. Even if it's perfectly true
with the MMC core code right now, does the MMC core document a guarantee
that this will always be true? Even if we optimize the WAR for the issue
as you've done, I think we should still have code that validates that
the same register is never written back-to-back to detect this likely
very hard-to-debug problem.
You're right - nothing in life is guaranteed. We had test code for
this. I'll add a config option (default on) that verifies back to back
writes do not occur.
+ * The BLOCK_SIZE and BLOCK_COUNT are meaningless until a command issued so
+ * the work around can be further optimized. We can keep shadow values of
+ * BLOCK_SIZE, BLOCK_COUNT, and TRANSFER_MODE until a COMMAND is issued.
+ * Then, write the BLOCK_SIZE+BLOCK_COUNT in a single 32-bit write followed
+ * by the TRANSFER+COMMAND in another 32-bit write.
*/
After this patch, the entire WAR for this issue is contained within
bcm2835_sdhci_writew(). It might be a good idea to move the comment next
to that function so it's more at hand to explain the code that's there.
Or at least add a comment to that function the to mention the location
of the explanation for the complex code.
ok, I'll clean up the comment a little more too.
static inline u32 bcm2835_sdhci_readl(struct sdhci_host *host, int reg)
{
u32 val = readl(host->ioaddr + reg);
@@ -71,76 +57,83 @@ static inline u32 bcm2835_sdhci_readl(struct sdhci_host *host, int reg)
return val;
}
-static void bcm2835_sdhci_writew(struct sdhci_host *host, u16 val, int reg)
-{
... (entire function deleted)
-}
This patch could be a lot smaller if it didn't re-order the functions at
the same time. It makes the patch harder to understand. If you must
re-order the functions, perhaps make that a separate patch that does
nothing else, so that the actual code changes are easier to see?
ok
static u16 bcm2835_sdhci_readw(struct sdhci_host *host, int reg)
{
- u32 val = bcm2835_sdhci_readl(host, (reg & ~3));
- u32 word_num = (reg >> 1) & 1;
- u32 word_shift = word_num * 16;
- u32 word = (val >> word_shift) & 0xffff;
-
+ u32 val = bcm2835_sdhci_readl(host->ioaddr, (reg & ~3));
The change from host to host->ioaddr ends up passing the wrong value to
bcm2835_sdhci_readl(). This causes the kernel to crash during boot.
I see that now. Will fix - unfortunately I ported from an existing
driver that doesn't need the bcm2835_shdci_readl function.
The compiler doesn't warn about this because host->ioaddr is void, so
can be automatically converted to struct sdhci_host *.
+ u16 word = val >> (reg << 3 & 0x18) & 0xffff;
return word;
}
To be honest, I think the existing code is a bit clearer, since it uses
variables with names to explain all the intermediate values. Assuming
the compiler is competent (which admittedly I haven't checked) I would
expect the same code to be generated either way, or at least something
pretty similar. Did you measure the benefit of the optimization?
By optimize I meant use the same bit calculation instead of doing
different calculations for the same operation. I'll create a macro to
make it clearer to see.
+static void bcm2835_sdhci_writew(struct sdhci_host *host, u16 val, int reg)
{
+ struct sdhci_pltfm_host *pltfm_host = sdhci_priv(host);
+ struct bcm2835_sdhci_host *bcm2835_host = pltfm_host->priv;
+ u32 word_shift = reg << 3 & 0x18;
+ u32 mask = 0xffff << word_shift;
+ u32 oldval;
+ u32 newval;
+
+ if (reg == SDHCI_COMMAND) {
+ if (bcm2835_host->shadow_blk != 0) {
+ writel(bcm2835_host->shadow_blk,
+ host->ioaddr + SDHCI_BLOCK_SIZE);
+ bcm2835_host->shadow_blk = 0;
+ }
Is it absolutely guaranteed that there's never a need to write 0 to that
register? I can see that no data transfer command is likely to transfer
0 blocks. I assume no other type of command uses that register as a
parameter?
Correct.
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