Subject: Re: [PATCH v3 4/6] powerpc/powernv: replace the hard coded boundary with gate

From: Wei Yang <[email-masked]>
Date: Mon, 17 Aug 2015 10:21:29 +0800

On Sat, Aug 15, 2015 at 08:27:04PM +1000, Alexey Kardashevskiy wrote:
>On 08/14/2015 12:11 AM, Wei Yang wrote:
>>At the moment 64bit-prefetchable window can be maximum 64GB, which is
>>currently got from device tree. This means that in shared mode the maximum
>>supported VF BAR size is 64GB/256=256MB. While this size could exhaust the
>>whole 64bit-prefetchable window. This is a design decision to set a
>>boundary to 64MB of the VF BAR size. Since VF BAR size with 64MB would
>>occupy a quarter of the 64bit-prefetchable window, this is affordable.
>>
>>This patch replaces magic limit of 64MB with (m64_segsize >> 1) and adds
>>comment to explain the reason for it.
>
>
>Having m64_segsize divided in _halves_ is also magic (or is it a
>"design decision"?).
>

Hmm... as the last sentence in previous paragraph said, 64MB VF BAR would
occupy a quarter of the 64bit-prefetchable window which is affordable.

Maybe I could change to this:

   This patch replaces magic limit of 64MB with "gate", which is 1/4 of the
   M64 Segment Size(m64_segsize >> 2) and adds comment to explain the reason
   for it.

This time I use (m64_segsize >> 2) instead of (m64_segsize >> 1), since I
decide to still use ">" instead of ">=". By doing so, the explanation in
commit log is consistent with the code. 

>
>>
>>Signed-off-by: Wei Yang <[email-masked]>
>>---
>>  arch/powerpc/platforms/powernv/pci-ioda.c |   22 +++++++++++++++++-----
>>  1 file changed, 17 insertions(+), 5 deletions(-)
>>
>>diff --git a/arch/powerpc/platforms/powernv/pci-ioda.c b/arch/powerpc/platforms/powernv/pci-ioda.c
>>index 4da0f50..3e8c0b4 100644
>>--- a/arch/powerpc/platforms/powernv/pci-ioda.c
>>+++ b/arch/powerpc/platforms/powernv/pci-ioda.c
>>@@ -2688,7 +2688,7 @@ static void pnv_pci_ioda_fixup_iov_resources(struct pci_dev *pdev)
>>  	struct pnv_phb *phb;
>>  	struct resource *res;
>>  	int i;
>>-	resource_size_t size;
>>+	resource_size_t size, gate;
>
>It should be:
>const resource_size_t gate = phb->ioda.m64_segsize >> 1;
>
>as it never changes in the function.

Good.

>
>
>>  	struct pci_dn *pdn;
>>  	int mul, total_vfs;
>>
>>@@ -2704,6 +2704,17 @@ static void pnv_pci_ioda_fixup_iov_resources(struct pci_dev *pdev)
>>
>>  	total_vfs = pci_sriov_get_totalvfs(pdev);
>>  	mul = phb->ioda.total_pe;
>>+	/*
>>+	 * If bigger than or equal to half of M64 segment size, just round up
>>+	 * power of two.
>>+	 *
>>+	 * Generally, one M64 BAR maps one IOV BAR. To avoid conflict with
>>+	 * other devices, IOV BAR size is expanded to be (total_pe *
>>+	 * VF_BAR_size).  When VF_BAR_size is half of M64 segment size , the
>>+	 * expanded size would equal to half of the whole M64 Space size,
>>+	 * which will exhaust the M64 Space and limit the system flexibility.
>>+	 */
>>+	gate = phb->ioda.m64_segsize >> 1;
>>
>>  	for (i = 0; i < PCI_SRIOV_NUM_BARS; i++) {
>>  		res = &pdev->resource[i + PCI_IOV_RESOURCES];
>>@@ -2718,10 +2729,11 @@ static void pnv_pci_ioda_fixup_iov_resources(struct pci_dev *pdev)
>>
>>  		size = pci_iov_resource_size(pdev, i + PCI_IOV_RESOURCES);
>>
>>-		/* bigger than 64M */
>>-		if (size > (1 << 26)) {
>>-			dev_info(&pdev->dev, "PowerNV: VF BAR%d: %pR IOV size is bigger than 64M, roundup power2\n",
>>-				 i, res);
>>+		/* bigger than or equal to gate */
>
>
>That multiline comment is better to be here, I think.
>

As you wish.

>
>>+		if (size >= gate) {
>>+			dev_info(&pdev->dev, "PowerNV: VF BAR%d: %pR IOV size "
>>+				"is bigger than %lld, roundup power2\n",
>>+				 i, res, gate);
>>  			mul = roundup_pow_of_two(total_vfs);
>>  			pdn->m64_single_mode = true;
>>  			break;
>>
>
>
>-- 
>Alexey

-- 
Richard Yang
Help you, Help me