linux-kernel - Re: Report in downstream Debian: mpt3sas broken with xen dom0 with update to 5.10.149 in 5.10.y.

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Date:   Mon, 24 Oct 2022 10:30:41 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Sreekanth Reddy <sreekanth.reddy@...adcom.com>
Cc:     Bart Van Assche <bvanassche@....org>,
        Salvatore Bonaccorso <carnil@...ian.org>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        sathya.prakash@...adcom.com, suganath-prabu.subramani@...adcom.com,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        MPT-FusionLinux.pdl@...adcom.com, linux-scsi@...r.kernel.org,
        linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
        adi@...egisch.at
Subject: Re: Report in downstream Debian: mpt3sas broken with xen dom0 with
 update to 5.10.149 in 5.10.y.

On Mon, Oct 24, 2022 at 05:26:44PM +0530, Sreekanth Reddy wrote:
> This issue is getting observed after having the below patch changes,
> 2b9aba0c5d58e141e32bb1bb4c7cd91d19f075b8 scsi: mpt3sas: Fix return
> value check of dma_get_required_mask()

Looking at this commit it seems odd.  dma_get_required_mask() should
only be used as an optimization for hardware that actually benefits
from a lower DMA Mask.  That means either classic PCI that requires
DAC cycles, or firmware architectures like aic7xxx that do need
additional overhead.  I don't think either is the case for mpt3sas,
so I think (in addition to fixing up the Xen required mask), mpt3sas
should do something like:

diff --git a/drivers/scsi/mpt3sas/mpt3sas_base.c b/drivers/scsi/mpt3sas/mpt3sas_base.c
index 4e981ccaac4163..295942a8989780 100644
--- a/drivers/scsi/mpt3sas/mpt3sas_base.c
+++ b/drivers/scsi/mpt3sas/mpt3sas_base.c
@@ -2992,8 +2992,7 @@ _base_config_dma_addressing(struct MPT3SAS_ADAPTER *ioc, struct pci_dev *pdev)
 	struct sysinfo s;
 	u64 coherent_dma_mask, dma_mask;
 
-	if (ioc->is_mcpu_endpoint || sizeof(dma_addr_t) == 4 ||
-	    dma_get_required_mask(&pdev->dev) <= DMA_BIT_MASK(32)) {
+	if (ioc->is_mcpu_endpoint) {
 		ioc->dma_mask = 32;
 		coherent_dma_mask = dma_mask = DMA_BIT_MASK(32);
 	/* Set 63 bit DMA mask for all SAS3 and SAS35 controllers */