Re: [PATCH 2.6.23-rc3] pata_pdc2027x: PLL detection fixes

[Jeff Garzik <jeff@garzik.org>]

Mikael Pettersson wrote:
> Previously I reported that the pata_pdc2027x PLL detection changes
> in kernel 2.6.22 broke the driver on my PowerMac:
> 
>> pata_pdc2027x: Invalid PLL input clock 1691742kHz, give up!
> 
> This is followed by a number of errors and speed reduction
> steps on the affected ports.
> 
> There are two bugs in pata_pdc2027x's PLL detection code:
> 
> 1. The PLL counter's start value is read before the chip is
>    put in "test mode". Outside of test mode the counter is
>    halted, and on the PowerMac the counter is zero because
>    the chip hasn't been initialised by its BIOS.
> 
>    The fix is to move the read of the start value to after
>    test mode is started, but before the mdelay() in test mode.
>    This also improves the precision of the PLL detection.
> 
> 2. The code to compute the number of PLL decrements during the
>    mdelay() in test mode fails to consider that the PLL counter
>    only is 30 bits wide. If there is a wraparound, it will compute
>    an incorrect and much too large value. On the PowerMac, the
>    start count is zero, the end count is a large 30-bit value, so
>    wraparound occurs and an out of bounds PLL clock is detected.
> 
>    The fix is to mask the (start - end) computation to 30 bits.
> 
> While debugging this I also noticed that pdc_read_counter()
> reads the two halves of the 30-bit PLL counter as 16-bit values,
> and then combines them as if the halves only are 15 bits wide.
> To avoid confusion, the halves should be read as 15-bit values.
> 
> This patch implements all three changes. It fixes the PLL detection
> failure on my PowerMac, and doesn't cause any regressions on an x86
> with an identical card.
> 
> Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>

Fantastic!  Thanks for putting in a great effort to track these down.

I'll queue it up [unless someone responds with a problem requiring 
revision, of course]


> diff -rupN linux-2.6.23-rc3/drivers/ata/pata_pdc2027x.c linux-2.6.23-rc3.pata_pdc2027x-pll-detection-fixes/drivers/ata/pata_pdc2027x.c
> --- linux-2.6.23-rc3/drivers/ata/pata_pdc2027x.c	2007-07-09 22:01:31.000000000 +0200
> +++ linux-2.6.23-rc3.pata_pdc2027x-pll-detection-fixes/drivers/ata/pata_pdc2027x.c	2007-08-18 21:53:40.000000000 +0200
> @@ -563,13 +563,13 @@ static long pdc_read_counter(struct ata_
>  	u32 bccrl, bccrh, bccrlv, bccrhv;
>  
>  retry:
> -	bccrl = readl(mmio_base + PDC_BYTE_COUNT) & 0xffff;
> -	bccrh = readl(mmio_base + PDC_BYTE_COUNT + 0x100) & 0xffff;
> +	bccrl = readl(mmio_base + PDC_BYTE_COUNT) & 0x7fff;
> +	bccrh = readl(mmio_base + PDC_BYTE_COUNT + 0x100) & 0x7fff;
>  	rmb();
>  
>  	/* Read the counter values again for verification */
> -	bccrlv = readl(mmio_base + PDC_BYTE_COUNT) & 0xffff;
> -	bccrhv = readl(mmio_base + PDC_BYTE_COUNT + 0x100) & 0xffff;
> +	bccrlv = readl(mmio_base + PDC_BYTE_COUNT) & 0x7fff;
> +	bccrhv = readl(mmio_base + PDC_BYTE_COUNT + 0x100) & 0x7fff;
>  	rmb();
>  
>  	counter = (bccrh << 15) | bccrl;

Unrelated to your changes, but, I wonder why those rmb() are there at 
all...?

	Jeff