linux-kernel - Re: [PATCH v9 25/27] PCI: dwc: Set coherent DMA-mask on MSI-address allocation

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Message-ID: <20230122003455.mftnxdsjyemxz7bb@mobilestation>
Date:   Sun, 22 Jan 2023 03:34:55 +0300
From:   Serge Semin <fancer.lancer@...il.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Serge Semin <Sergey.Semin@...kalelectronics.ru>,
        Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
        Vinod Koul <vkoul@...nel.org>, Rob Herring <robh@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Cai Huoqing <cai.huoqing@...ux.dev>,
        Robin Murphy <robin.murphy@....com>,
        Jingoo Han <jingoohan1@...il.com>, Frank Li <Frank.Li@....com>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Pavel Parkhomenko <Pavel.Parkhomenko@...kalelectronics.ru>,
        caihuoqing <caihuoqing@...du.com>,
        Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
        linux-pci@...r.kernel.org, dmaengine@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 25/27] PCI: dwc: Set coherent DMA-mask on MSI-address
 allocation

On Fri, Jan 20, 2023 at 03:59:52PM -0600, Bjorn Helgaas wrote:
> On Fri, Jan 13, 2023 at 08:14:07PM +0300, Serge Semin wrote:
> > The MSI target address requires to be reserved within the lowest 4GB
> > memory in order to support the PCIe peripherals with no 64-bit MSI TLPs
> > support. Since the allocation is done from the DMA-coherent memory let's
> > modify the allocation procedure to setting the coherent DMA-mask only and
> > avoiding the streaming DMA-mask modification. Thus at least the streaming
> > DMA operations would work with no artificial limitations. It will be
> > specifically useful for the eDMA-capable controllers so the corresponding
> > DMA-engine clients would map the DMA buffers with no need in the SWIOTLB
> > intervention for the buffers allocated above the 4GB memory region.
> > 
> > While at it let's add a brief comment about the reason of having the MSI
> > target address allocated from the DMA-coherent memory limited with the 4GB
> > upper bound.
> > 
> > Signed-off-by: Serge Semin <Sergey.Semin@...kalelectronics.ru>
> > Reviewed-by: Robin Murphy <robin.murphy@....com>
> > 
> > ---
> > 
> > Changelog v8:
> > - This is a new patch added on v8 stage of the series.
> >   (@Robin, @Christoph)
> > ---
> >  drivers/pci/controller/dwc/pcie-designware-host.c | 11 ++++++++++-
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/pci/controller/dwc/pcie-designware-host.c b/drivers/pci/controller/dwc/pcie-designware-host.c
> > index 3ab6ae3712c4..e10608af39b4 100644
> > --- a/drivers/pci/controller/dwc/pcie-designware-host.c
> > +++ b/drivers/pci/controller/dwc/pcie-designware-host.c
> > @@ -366,7 +366,16 @@ static int dw_pcie_msi_host_init(struct dw_pcie_rp *pp)
> >  						    dw_chained_msi_isr, pp);
> >  	}
> >  
> > -	ret = dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32));
> > +	/*
> > +	 * Even though the iMSI-RX Module supports 64-bit addresses some
> > +	 * peripheral PCIe devices may lack the 64-bit messages support. In
> > +	 * order not to miss MSI TLPs from those devices the MSI target address
> > +	 * has to be reserved within the lowest 4GB.
> > +	 * Note until there is a better alternative found the reservation is
> > +	 * done by allocating from the artificially limited DMA-coherent
> > +	 * memory.
> > +	 */
> > +	ret = dma_set_coherent_mask(dev, DMA_BIT_MASK(32));
> 

> We used to restrict both streaming and coherent DMA masks to 32 bits.
> Now we will only restrict coherent DMA to 32 bits.
> 
> So in essence this change removes a restriction on the streaming DMA
> mask, right?

Right.

> 
> And I guess bt1 is the only driver where this will make a difference
> (after the next patch) because no other dwc drivers set their own DMA
> masks?

Right. But that's only because I am sure the Baikal-T1 PCIe eDMA
engine is able to reach memory above 4GB limits. If you know any other
DW PCIe RP/EP controller with eDMA-capability and which can work with
the memory space wider than 4GB then you can update it' streaming
DMA-mask too. Note lacking to do so doesn't mean the eDMA-engine
driver won't work for them. The eDMA-engine will be forced to work
with the lowest 4GB space to which the upper memory will be mapped via
SWIOTLB.

-Serge(y)

> 
> >  	if (ret)
> >  		dev_warn(dev, "Failed to set DMA mask to 32-bit. Devices with only 32-bit MSI support may not work properly\n");
> >  
> > -- 
> > 2.39.0
> > 
> >