linux-kernel - Re: [PATCH 0/3] [SCSI] megaraid: Remove local (struct pci

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Date:	Tue, 09 Jul 2013 22:18:15 +0100
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Myron Stowe <myron.stowe@...hat.com>,
	adam radford <aradford@...il.com>
Cc:	megaraidlinux@....com, linux-scsi@...r.kernel.org,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] [SCSI] megaraid: Remove local (struct pci_dev)
 pdev's

On Tue, 2013-07-09 at 14:39 -0600, Myron Stowe wrote:
> Is the "megaraid" driver still actively used and maintained?  I originally
> posted this series on 06.07.2013 and after receiving no comments, pinged
> the list again on 06.17.2013 and still received no comments/feedback.
> 
> Trying again as I believe there is a real issue here, which I'd like
> confirmation on, and we really should remove the local copy/usage of
> 'struct pci_dev' that this driver currently maintains.
> 
> 
> While the megaraid device itself may be 64-bit DMA capable, 32-bit address
> restricted DMA buffers are apparently required for "internal commands" as
> is denoted by a couple of comments - "For all internal commands, the
> buffer must be allocated in <4GB address range" - within the driver.
> 
> If the device is 64-bit DMA capable then, once it is setup, any subsequent
> DMA allocations for "internal commands" would not be properly restricted
> due to megaraid_probe_one() having called pci_set_dma_mask() on pdev with
> DMA_BIT_MASK(64).  The driver attempts to solve this by using
> make_local_pdev() to dynamically create local pci_dev structures which are
> then set and used for allocating 32-bit address space restricted DMA
> buffers[1] but I don't believe that the implementation works as intended.
> 
> 
> Assume that the megaraid device is 64-bit DMA capable.  While probing the
> device and attaching the megaraid driver, pci_set_dma_mask() is called
> with the "originating pdev" and a DMA_BIT_MASK of 64.  As a result, any
> subsequent dynamic DMA related allocations associated with the
> "originating pdev" will acquire 64-bit based buffers, which do not meet
> the addressing restrictions for internal commands.
> 
>   megaraid_probe_one(struct pci_dev *pdev, ...)
>       ...
>       pci_set_dma_mask(pdev, DMA_BIT_MASK(64));
> 
> As mentioned, the driver attempts to solve this by using make_local_pdev()
> to dynamically create local pci_dev structures - "local pdev's" - which
> are set with a DMA_BIT_MASK of 32.
> 
>   make_local_pdev
>       alloc_pci_dev
>       memcpy
>       pci_set_dma_mask
>           dma_set_mask
>               *dev->dma_mask = mask;
> 
> The "local pdev" is then used in allocating a DMA buffer in an attempt to
> meet the < 4 GB restriction.
> 
> For a 64-bit DMA capable device, the "originating pdev" will have its
> 'dma_mask' set to 0xffffffffffffffff after the driver attaches.
> Subsequently, when an internal command is initiated, make_local_pdev() is
> called.  make_local_pdev() uses the PCI's core to allocate a "local pdev"
> and then copies the "originating pdev" content into the newly allocated
> "local pdev".  As a result of copying the "originating pdev" content into
> the "local pdev", pdev->dev.dma_mask will be pointing back to the
> "originating pdev's" 'dma_mask' member, not the "local pdev's" as
> intended.  Thus, when make_local_pdev() calls pci_set_dma_mask() in an
> attempt to set the "local pdev's" DMA mask to 32 it will instead overwrite
> the "originating pdev's" DMA mask.  Thus, after any user initiated
> commands are issued, all subsequent DMA allocations will be 32-bit
> restricted from that point onward regardless of whether they are internal
> commands or otherwise.
> 
> 
> This patch fixes the issue by removing the setup of DMA_BIT_MASK to 64 in
> megaraid_probe_one(), leaving the driver with default 32-bit DMA
> capabilities, as it currently ends up in such a state anyway after any
> internal commands are initiated.
> 
> 
> [1] It seems strange that both mega_buffer/buf_dma_handle and
>     make_local_pdev() both exist for internal commands but this has been
>     the case for a long time - at least since 2.6.12-rc2.  Perhaps there
>     is some coalescing that could be done.
> ---
> Myron Stowe (3):
>       [SCSI] megaraid: Remove 64-bit DMA related dead code
>       [SCSI] megaraid: Remove local pdev's
>       [SCSI] megaraid: Remove 64-bit DMA_BIT_MASK capability

Adam, you do drive by coding on this for LSI ... ack or reject, please.

James


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