unchecked_isa_dma and BusLogic SCSI controller

unchecked_isa_dma and BusLogic SCSI controller

[Jun Sun]

Can someone enlighten me on what "unchecked_isa_dma" means in the
struct scsi_host_template?  Specifically why Bus_Logic_template has
it set to 1?

I am trying to reserve a block of memory (>16MB) starting from 0 and hide
it from kernel.  As a result, the DMA zone becomes 0 in size.

Because Bus_Logic_template has unchecked_isa_dma set to 1, the driver
will attempt to allocate a block of memory from DMA zone and thus
causes OOMs during its initialization.

It is hard for me to see why BusLogic controller would only do DMA
in low 16MB.  Is there a fix for this?

BTW, I also tried to increase MAX_DMA_ADDRESS to cover the whole memory
area.  While the OOMs are gone during BusLogic driver initialization, 
kernel fails to find labelled root partition or fail to open
the initial console.  It appears the disk (or the scsi) is not working
properly after increasing MAX_DMA_ADDRESS.

My platform is vmplayer.  Pretty cool for devel.

Cheers.

Jun

Re: unchecked_isa_dma and BusLogic SCSI controller

[Randy Dunlap]

On Wed, 1 Nov 2006 15:53:30 -0800 Jun Sun wrote:

> 
> Can someone enlighten me on what "unchecked_isa_dma" means in the
> struct scsi_host_template?

It's documented in Documentation/scsi/scsi_mid_low_api.txt:

    unchecked_isa_dma - 1=>only use bottom 16 MB of ram (ISA DMA addressing
                   restriction), 0=>can use full 32 bit (or better) DMA
                   address space

> Specifically why Bus_Logic_template has
> it set to 1?

maybe ask on linux-scsi@vger.kernel.org (cc-ed)

> I am trying to reserve a block of memory (>16MB) starting from 0 and hide
> it from kernel.  As a result, the DMA zone becomes 0 in size.
> 
> Because Bus_Logic_template has unchecked_isa_dma set to 1, the driver
> will attempt to allocate a block of memory from DMA zone and thus
> causes OOMs during its initialization.
> 
> It is hard for me to see why BusLogic controller would only do DMA
> in low 16MB.  Is there a fix for this?

Does anyone know that controller hardware and its limitations?

If the driver can't handle highmem addresses, you could just change
it so that
(a) it does not set unchecked_isa_dma
(b) it sets .slave_alloc in the host template and then does
like the <slave_alloc> function in ppa.c or imm.c:
call blk_queue_bounce_limit() to set an address for which bounce
buffers (from highmem to lowmem) will be used.
[I don't guarantee that to work for BusLogic.]

> BTW, I also tried to increase MAX_DMA_ADDRESS to cover the whole memory
> area.  While the OOMs are gone during BusLogic driver initialization, 
> kernel fails to find labelled root partition or fail to open
> the initial console.  It appears the disk (or the scsi) is not working
> properly after increasing MAX_DMA_ADDRESS.
> 
> My platform is vmplayer.  Pretty cool for devel.
> 
> Cheers.
> 
> Jun

---
~Randy

Re: unchecked_isa_dma and BusLogic SCSI controller

[Matthew Wilcox]

On Wed, Nov 01, 2006 at 05:33:58PM -0800, Randy Dunlap wrote:
>     unchecked_isa_dma - 1=>only use bottom 16 MB of ram (ISA DMA addressing
>                    restriction), 0=>can use full 32 bit (or better) DMA
>                    address space
> 
> > It is hard for me to see why BusLogic controller would only do DMA
> > in low 16MB.  Is there a fix for this?
> 
> Does anyone know that controller hardware and its limitations?

I don't, but:

		if (pci_set_dma_mask(PCI_Device, DMA_32BIT_MASK ))
			continue;

So somebody thinks the device can do 32-bit addressing.  I would expect
that setting unchecked_isa_dma is a historical mistake.  However, I
don't have any cards of this type to test.

Re: unchecked_isa_dma and BusLogic SCSI controller

[Maciej W. Rozycki]

On Thu, 2 Nov 2006, Matthew Wilcox wrote:

> > > It is hard for me to see why BusLogic controller would only do DMA
> > > in low 16MB.  Is there a fix for this?
> > 
> > Does anyone know that controller hardware and its limitations?
> 
> I don't, but:
> 
> 		if (pci_set_dma_mask(PCI_Device, DMA_32BIT_MASK ))
> 			continue;
> 
> So somebody thinks the device can do 32-bit addressing.  I would expect
> that setting unchecked_isa_dma is a historical mistake.  However, I
> don't have any cards of this type to test.

 It could be related to the non-PCI variations of the board.

  Maciej

Re: unchecked_isa_dma and BusLogic SCSI controller

[Christoph Hellwig]

On Thu, Nov 02, 2006 at 05:41:28AM -0700, Matthew Wilcox wrote:
> On Wed, Nov 01, 2006 at 05:33:58PM -0800, Randy Dunlap wrote:
> >     unchecked_isa_dma - 1=>only use bottom 16 MB of ram (ISA DMA addressing
> >                    restriction), 0=>can use full 32 bit (or better) DMA
> >                    address space
> > 
> > > It is hard for me to see why BusLogic controller would only do DMA
> > > in low 16MB.  Is there a fix for this?
> > 
> > Does anyone know that controller hardware and its limitations?
> 
> I don't, but:
> 
> 		if (pci_set_dma_mask(PCI_Device, DMA_32BIT_MASK ))
> 			continue;
> 
> So somebody thinks the device can do 32-bit addressing.  I would expect
> that setting unchecked_isa_dma is a historical mistake.  However, I
> don't have any cards of this type to test.

The buslogic driver also supports non-PCI devices, and at least the
ISA boards have the obvious limitiation that unchecked_isa_dma = 1
caters to.  Now someone would have to audit the driver whether it's
enough to only set this on the actual isa hosts instead of the host
template.