Date:   Fri, 30 Sep 2022 12:01:58 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Serge Semin <fancer.lancer@...il.com>
Cc:     Will McVicker <willmcvicker@...gle.com>,
        Jingoo Han <jingoohan1@...il.com>,
        Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>, kernel-team@...roid.com,
        Vidya Sagar <vidyas@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Isaac J . Manjarres" <isaacmanjarres@...gle.com>
Subject: Re: [PATCH v5 1/2] PCI: dwc: Drop dependency on ZONE_DMA32

On 2022-09-29 20:32, Serge Semin wrote:
> On Thu, Sep 29, 2022 at 07:25:03PM +0100, Robin Murphy wrote:
>> On 2022-09-28 12:41, Serge Semin wrote:
>>> On Thu, Aug 25, 2022 at 06:50:24PM +0000, Will McVicker wrote:
>>>> Re-work the msi_msg DMA allocation logic to use dmam_alloc_coherent() which
>>>> uses the coherent DMA mask to try to return an allocation within the DMA
>>>> mask limits. With that, we now can drop the msi_page parameter in struct
>>>> dw_pcie_rp. This allows kernel configurations that disable ZONE_DMA32 to
>>>> continue supporting a 32-bit DMA mask. Without this patch, the PCIe host
>>>> device will fail to probe when ZONE_DMA32 is disabled.
>>>
>>> As Rob already said here
>>> https://lore.kernel.org/all/CAL_JsqJh=d-B51b6yPBRq0tOwbChN=AFPr-a19U1QdQZAE7c1A@mail.gmail.com/
>>> and I mentioned in this thread
>>> https://lore.kernel.org/linux-pci/20220912000211.ct6asuhhmnatje5e@mobilestation/
>>> DW PCIe MSI doesn't cause any DMA due to the way the iMSI-RX engine is
>>> designed. So reserving any real system memory is a waste of one in
>>> this case. Reserving DMA-coherent even more inappropriate since it
>>> can be expensive on some platforms (see note in Part Ia of
>>> Documentation/core-api/dma-api.rst). For instance on MIPS32 with
>>> non-corehent common DMA.
>>
> 
>> This has been discussed before - in general it is difficult to pick an
>> arbitrary MSI address that is *guaranteed* not to overlap any valid DMA
>> address that somebody may try to use later. However there is a very easy way
>> to guarantee that the DMA API won't give anyone a particular DMA address,
>> which is to get an address directly from the DMA API and keep it. Yes, that
>> can technically be done with a streaming mapping *if* you already have some
>> memory allocated in a suitable physical location, but coherent allocations
>> are even more foolproof, simpler to clean up (particularly with devres), and
>> unlikely to be an issue on relevant platforms (do any MIPS32 systems use
>> this driver?)
> 
> My patchset adds the DW PCIe RP controller support on MIPS32 arch:
> https://lore.kernel.org/linux-pci/20220822184701.25246-21-Sergey.Semin@baikalelectronics.ru/
> 
>>
>>>> Fixes: 35797e672ff0 ("PCI: dwc: Fix MSI msi_msg DMA mapping")
>>>> Reported-by: Isaac J. Manjarres <isaacmanjarres@...gle.com>
>>>> Signed-off-by: Will McVicker <willmcvicker@...gle.com>
>>>> Acked-by: Jingoo Han <jingoohan1@...il.com>
>>>> Reviewed-by: Rob Herring <robh@...nel.org>
>>>> ---
>>>>    .../pci/controller/dwc/pcie-designware-host.c | 28 +++++--------------
>>>>    drivers/pci/controller/dwc/pcie-designware.h  |  1 -
>>>>    2 files changed, 7 insertions(+), 22 deletions(-)
>>>>
>>>> diff --git a/drivers/pci/controller/dwc/pcie-designware-host.c b/drivers/pci/controller/dwc/pcie-designware-host.c
>>>> index 7746f94a715f..39f3b37d4033 100644
>>>> --- a/drivers/pci/controller/dwc/pcie-designware-host.c
>>>> +++ b/drivers/pci/controller/dwc/pcie-designware-host.c
>>>> @@ -267,15 +267,6 @@ static void dw_pcie_free_msi(struct dw_pcie_rp *pp)
>>>>    	irq_domain_remove(pp->msi_domain);
>>>>    	irq_domain_remove(pp->irq_domain);
>>>> -
>>>> -	if (pp->msi_data) {
>>>> -		struct dw_pcie *pci = to_dw_pcie_from_pp(pp);
>>>> -		struct device *dev = pci->dev;
>>>> -
>>>> -		dma_unmap_page(dev, pp->msi_data, PAGE_SIZE, DMA_FROM_DEVICE);
>>>> -		if (pp->msi_page)
>>>> -			__free_page(pp->msi_page);
>>>> -	}
>>>>    }
>>>>    static void dw_pcie_msi_init(struct dw_pcie_rp *pp)
>>>> @@ -336,6 +327,7 @@ static int dw_pcie_msi_host_init(struct dw_pcie_rp *pp)
>>>>    	struct dw_pcie *pci = to_dw_pcie_from_pp(pp);
>>>>    	struct device *dev = pci->dev;
>>>>    	struct platform_device *pdev = to_platform_device(dev);
>>>> +	u64 *msi_vaddr;
>>>>    	int ret;
>>>>    	u32 ctrl, num_ctrls;
>>>> @@ -375,22 +367,16 @@ static int dw_pcie_msi_host_init(struct dw_pcie_rp *pp)
>>>>    						    dw_chained_msi_isr, pp);
>>>>    	}
>>>
>>>> -	ret = dma_set_mask(dev, DMA_BIT_MASK(32));
>>>> +	ret = dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32));
>>>
>>> This has been redundant in the first place since none of the DW PCIe
>>> low-level drivers update the mask, and it's of 32-bits wide by default
>>> anyway:
>>> https://elixir.bootlin.com/linux/latest/source/drivers/of/platform.c#L167
>>
> 
>> No, in general drivers should always explicitly set their mask(s) and check
>> the return value to make sure DMA is possible at all before trying any other
>> DMA API calls. There's no guarantee that the default mask is usable (e.g.
>> some systems don't have any 32-bit addressable RAM), or that it's even
>> always 32 bits (due to crufty reasons of something of_dma_configure() tried
>> to do a long time ago).
> 
> Suppose you are right and DMA-mask should be always set before any
> mapping. What do you suggest to do in this case? (1) The code above
> overrides the real DMA-mask which could be set by the platform
> drivers, which in its turn are normally aware of the device DMA
> capabilities.

I am right. Appropriate DMA API usage as defined by the DMA API 
maintainers is not a matter of supposition. I literally just explained 
right there why drivers can't blindly assume the default mask is usable 
on modern systems (yes, it was different 20 years ago when system 
topologies were simpler).

However, having now gone and looked at the whole driver rather than 
unclear fragments of patch context, the code here *is* technically 
wrong. I've been mistakenly thinking all along that this was operating 
on the PCI device because I know that's what it *should* be doing, and 
seeing misleading things like "dev = pci->dev" falsely affirmed that 
assumption that it would be correct because it's been around for ages. 
AFAIU the correct PCI device won't actually exist until we've got far 
enough through pci_host_probe(), so I'm not sure how to easily solve this :/

Of course *this* patch doesn't change any of that either, so it's no 
worse than the existing code and I don't see that dropping it helps you 
at all; the current driver is already trampling your 64-bit mask back to 
32 bits and reserving the doorbell address in the wrong DMA address 
space (modulo the other dma-ranges bug which also took far too long to 
figure out). At this point I'd rather keep it since getting rid of the 
__GFP_DMA32 abuse is objectively good. If losing one page of coherent 
memory is a measurably significant problem for T1 once the other issues 
are worked out and that series lands, then you're welcome to propose a 
change on top (but I would prefer that all the drivers using this trick 
are changed consistently).

Thanks,
Robin.

> But in this case due to override afterwards any buffers
> above 4GB mapping will cause using the bounce buffers. (2) It's set
> here for something which isn't actual DMA. So to speak on one side is
> this patchset which overrides the mask for something which isn't DMA,
> and there are another patchsets:
> https://lore.kernel.org/linux-pci/20220822184701.25246-1-Sergey.Semin@baikalelectronics.ru/
> and
> https://lore.kernel.org/linux-pci/20220728142841.12305-1-Sergey.Semin@baikalelectronics.ru/
> which add the real DMA support to DW PCIe driver and for which setting
> the real DMA-mask is crucial. What do you suggest? Setting the mask
> twice: before allocating MSI-buffer and afterwards for the sake of
> eDMA buffers mapping? Moving DMA-mask setting from the generic DW PCIe
> code to the platform drivers?
> 
> -Sergey
> 
>>
>> Thanks,
>> Robin.
>>
>>>>    	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");
>>>> -	pp->msi_page = alloc_page(GFP_DMA32);
>>>> -	pp->msi_data = dma_map_page(dev, pp->msi_page, 0,
>>>> -				    PAGE_SIZE, DMA_FROM_DEVICE);
>>>> -	ret = dma_mapping_error(dev, pp->msi_data);
>>>> -	if (ret) {
>>>> -		dev_err(pci->dev, "Failed to map MSI data\n");
>>>> -		__free_page(pp->msi_page);
>>>> -		pp->msi_page = NULL;
>>>> -		pp->msi_data = 0;
>>>> +	msi_vaddr = dmam_alloc_coherent(dev, sizeof(u64), &pp->msi_data,
>>>> +					GFP_KERNEL);
>>>
>>> Changing the whole device DMA-mask due to something that doesn't
>>> perform seems inappropriate. I'd suggest to preserve the ZONE_DMA32
>>> here until there is something like suggested by @Robin
>>> https://lore.kernel.org/linux-pci/1e63a581-14ae-b4b5-a5bf-ca8f09c33af6@arm.com/
>>> in the last paragraph is implemented. Especially seeing there still
>>> common drivers in kernel which still rely on that zone.
>>>
>>> -Sergey
>>>
>>>> +	if (!msi_vaddr) {
>>>> +		dev_err(dev, "Failed to alloc and map MSI data\n");
>>>>    		dw_pcie_free_msi(pp);
>>>> -
>>>> -		return ret;
>>>> +		return -ENOMEM;
>>>>    	}
>>>>    	return 0;
>>>> diff --git a/drivers/pci/controller/dwc/pcie-designware.h b/drivers/pci/controller/dwc/pcie-designware.h
>>>> index 09b887093a84..a871ae7eb59e 100644
>>>> --- a/drivers/pci/controller/dwc/pcie-designware.h
>>>> +++ b/drivers/pci/controller/dwc/pcie-designware.h
>>>> @@ -243,7 +243,6 @@ struct dw_pcie_rp {
>>>>    	struct irq_domain	*irq_domain;
>>>>    	struct irq_domain	*msi_domain;
>>>>    	dma_addr_t		msi_data;
>>>> -	struct page		*msi_page;
>>>>    	struct irq_chip		*msi_irq_chip;
>>>>    	u32			num_vectors;
>>>>    	u32			irq_mask[MAX_MSI_CTRLS];
>>>> -- 
>>>> 2.37.2.672.g94769d06f0-goog
>>>>
>>>>

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