Re: Proposal for new generic device API: dma_get_required_mask()

Matthew Wilcox
2004-06-17T15:28:19Z

On Thu, Jun 17, 2004 at 09:08:51AM -0500, James Bottomley wrote:
> We have a large number of devices in scsi: aacraid, aic7xxx, qla1280,
> qla2xxx which can all do full 64 bit DMA, but which pay a performance
> penalty for using the larger descriptors (aic7xxx is stranger in that it
> has three modes of operation: 32 bit, 39 bit and 64 bit each with an
> increasing performance penalty).

sym2 too -- 32, 40 and 64-bit, but with the restriction of the 64-bit
mode that it's limited to accessing 16 4GB segments.

> Once the driver has the platform's optimal mask, it can use this to
> decide on the correct descriptor size.

Sounds good to me.

So let's look at the sym2 implementation to see how this would work.
A straight diff -u is not terribly helpful, So let's try a more verbose
context diff:

*** 1625,1657 ****
*/
static int sym_setup_bus_dma_mask(struct sym_hcb *np)
{
! #if SYM_CONF_DMA_ADDRESSING_MODE == 0
! if (pci_set_dma_mask(np->s.device, 0xffffffffUL))
! goto out_err32;
! #else
! #if SYM_CONF_DMA_ADDRESSING_MODE == 1
! #define PciDmaMask 0xffffffffffULL
! #elif SYM_CONF_DMA_ADDRESSING_MODE == 2
! #define PciDmaMask 0xffffffffffffffffULL
! #endif
if (np->features & FE_DAC) {
! if (!pci_set_dma_mask(np->s.device, PciDmaMask)) {
! np->use_dac = 1;
! printf_info("%s: using 64 bit DMA addressing\n",
! sym_name(np));
! } else {
! if (pci_set_dma_mask(np->s.device, 0xffffffffUL))
! goto out_err32;
! }
}
! #undef PciDmaMask
! #endif
return 0;

! out_err32:
! printf_warning("%s: 32 BIT DMA ADDRESSING NOT SUPPORTED\n",
! sym_name(np));
! return -1;
}

/*
--- 1625,1645 ----
*/
static int sym_setup_bus_dma_mask(struct sym_hcb *np)
{
! struct pci_dev *pdev = &np->s.device;
!
if (np->features & FE_DAC) {
! u64 required_mask = dma_set_mask(&pdev->dev, 0xffffffffffULL);
! if (required_mask > 0xffffffffUL)
! goto use_dac;
}
!
! dma_set_mask(&pdev->dev, 0xffffffffUL);
return 0;

! use_dac:
! np->use_dac = 1;
! printf_info("%s: using 64 bit DMA addressing\n", sym_name(np));
! return 0;
}

/*

There's an obvious problem here -- no error checking on the second
dma_set_mask(). How were we going to indicate "I can't do that"
errors again?

--
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the nation that is attacked, and every man will be glad of those
conscience-soothing falsities, and will diligently study them, and refuse
to examine any refutations of them; and thus he will by and by convince
himself that the war is just, and will thank God for the better sleep
he enjoys after this process of grotesque self-deception." -- Mark Twain