On 2/12/2018 9:59 PM, Michael Ellerman wrote:
Johannes Thumshirn <jthumsh...@suse.de> writes:

On Wed, Feb 07, 2018 at 10:51:57AM +0100, Johannes Thumshirn wrote:
+                       /* Enable combined writes for DPP aperture */
+                       pg_addr = (unsigned long)(wq->dpp_regaddr) & PAGE_MASK;
+#ifdef CONFIG_X86
+                       rc = set_memory_wc(pg_addr, 1);
+                       if (rc) {
+                               lpfc_printf_log(phba, KERN_ERR, LOG_INIT,
+                                               "3272 Cannot setup Combined "
+                                               "Write on WQ[%d] - disable 
+                                               wq->queue_id);
+                               phba->cfg_enable_dpp = 0;
+                       }
+                       phba->cfg_enable_dpp = 0;
+               } else
+                       wq->db_regaddr = phba->sli4_hba.WQDBregaddr;

I don't really like the set_memory_wc() call here. Neither do I like the ifdef
CONFIG_X86 special casing.

If you really need write combining, can't you at least use ioremap_wc()?

Coming back to this again (after talking to our ARM/POWER folks internally).
Is this really x86 specific here? I know there are servers with other 
using lpfcs out there.

I _think_ write combining should be possible on other architectures (that have
PCIe and aren't dead) as well.

The ioremap_wc() I suggested is probably wrong.

So can you please revisit this? I CCed Mark and Michael, maybe they can help

I'm not much of an I/O guy, but I do know that on powerpc we don't
implement set_memory_wc(). So if you're using that then you do need the

I couldn't easily find the rest of this thread, so I'm not sure if
ioremap_wc() is an option. We do implement that and on modern CPUs at
least it will give you something that's not just a plain uncached

I went back and looked at things. It does appear that we should be using ioremap_wc(). There's a pci routine that wrappers it, but as we're already are using the other routines in the wrapper, it's not very interesting. Ioremap_wc seems to be supported pretty much anywhere, with platforms managing what it resolves to. Granted, some platforms may not do write combining but will relax the caching aspects (as Michael indicates).

The interesting thing is - when wc is truly on, we see a substantial difference. But in cases where wc isn't on and we perform the individual writes plus the flush before the doorbell write to synchronize things, it turns out it takes longer than if we don't use the feature. So, in cases where we don't have real wc, I'm going to turn it off. Based on what we've tested so far (includes ppc p8), we'll be leaving it enabled on X86 only.

-- james

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