linux-kernel - Re: [PATCH v4 7/7] PCI: endpoint: Add PCI Endpoint function driver for MHI bus

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Date:   Tue, 30 May 2023 16:48:53 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To:     Damien Le Moal <dlemoal@...nel.org>
Cc:     Manivannan Sadhasivam <mani@...nel.org>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>, kw@...ux.com,
        kishon@...nel.org, bhelgaas@...gle.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH v4 7/7] PCI: endpoint: Add PCI Endpoint function driver
 for MHI bus

On Tue, May 30, 2023 at 08:37:07AM +0900, Damien Le Moal wrote:
> On 5/30/23 02:38, Manivannan Sadhasivam wrote:
> [...]
> >>> +static void pci_epf_mhi_unmap_free(struct mhi_ep_cntrl *mhi_cntrl, u64 pci_addr,
> >>> +				   phys_addr_t phys_addr, void __iomem *virt_addr, size_t size)
> >>> +{
> >>> +	struct pci_epf_mhi *epf_mhi = container_of(mhi_cntrl, struct pci_epf_mhi, mhi_cntrl);
> >>> +	struct pci_epf *epf = epf_mhi->epf;
> >>> +	struct pci_epc *epc = epf->epc;
> >>> +	size_t offset = pci_addr & (epc->mem->window.page_size - 1);
> >>> +
> >>> +	pci_epc_unmap_addr(epc, epf->func_no, epf->vfunc_no, phys_addr - offset);
> >>> +	pci_epc_mem_free_addr(epc, phys_addr - offset, virt_addr - offset, size + offset);
> >>> +}
> >>> +
> >>> +static void pci_epf_mhi_raise_irq(struct mhi_ep_cntrl *mhi_cntrl, u32 vector)
> >>> +{
> >>> +	struct pci_epf_mhi *epf_mhi = container_of(mhi_cntrl, struct pci_epf_mhi, mhi_cntrl);
> >>> +	struct pci_epf *epf = epf_mhi->epf;
> >>> +	struct pci_epc *epc = epf->epc;
> >>> +
> >>> +	/*
> >>> +	 * Vector is incremented by 1 here as the DWC core will decrement it before
> >>> +	 * writing to iATU.
> >>
> >> This isn't OK. It is an API, you can't write code explicitly relying on
> >> the underlying implementation. I assume the API is not well specified,
> >> that's why we need these tricks ?
> >>
> > 
> > Well, this is not an API issue but rather an implementation detail of the DWC EP
> > core driver. The DWC driver expects the interrupt vectors to be 1 based, so it
> > decrements it before writing to the MSI address:
> > [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/controller/dwc/pcie-designware-ep.c#n537
> 
> Then the driver should be fixed so that this peculiarity is not visible at the
> user API level, resulting in a uniform usage of the API for all functions
> regardless of the controller being used.
> 

Just checked with Kishon offline and confirmed that this is an API behavior.
I also checked other drivers and they all doing the same.

But unfortunately, this behavior is not documented in the API. Will add a patch
for that.

- Mani

> -- 
> Damien Le Moal
> Western Digital Research
> 

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