Re: [PATCH 2/5] misc: zynq: Add afi config driver

[Greg KH]

On Tue, Apr 20, 2021 at 01:36:51PM +0000, Nava kishore Manne wrote:
> Hi Greg,
> 
> 	Please find my response inline.
> 
> > -----Original Message-----
> > From: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
> > Sent: Tuesday, April 20, 2021 2:17 PM
> > To: Nava kishore Manne <navam@xxxxxxxxxx>
> > Cc: robh+dt@xxxxxxxxxx; Michal Simek <michals@xxxxxxxxxx>; Derek Kiernan
> > <dkiernan@xxxxxxxxxx>; Dragan Cvetic <draganc@xxxxxxxxxx>;
> > arnd@xxxxxxxx; Rajan Vaja <RAJANV@xxxxxxxxxx>; Jolly Shah
> > <JOLLYS@xxxxxxxxxx>; Tejas Patel <tejasp@xxxxxxxxxxxxxxx>; Amit Sunil
> > Dhamne <amitsuni@xxxxxxxxxx>; devicetree@xxxxxxxxxxxxxxx; linux-arm-
> > kernel@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> > chinnikishore369@xxxxxxxxx; git <git@xxxxxxxxxx>
> > Subject: Re: [PATCH 2/5] misc: zynq: Add afi config driver
> > 
> > On Tue, Apr 20, 2021 at 01:41:50PM +0530, Nava kishore Manne wrote:
> > > This patch adds zynq afi config driver. This is useful for the
> > > configuration of the PS-PL interface on zynq platform.
> > 
> > What is "PS-PL"?  Can you describe it better please?
> > 
> PS-PL interface is nothing but the interface between processing system(PS)  that contains arm cores and Programmable Logic(PL) i.e FPGA.
> Will update the description in v2.
> 
> > >
> > > Signed-off-by: Nava kishore Manne <nava.manne@xxxxxxxxxx>
> > > ---
> > >  drivers/misc/Kconfig    | 11 ++++++
> > >  drivers/misc/Makefile   |  1 +
> > >  drivers/misc/zynq-afi.c | 81
> > > +++++++++++++++++++++++++++++++++++++++++
> > >  3 files changed, 93 insertions(+)
> > >  create mode 100644 drivers/misc/zynq-afi.c
> > >
> > > diff --git a/drivers/misc/Kconfig b/drivers/misc/Kconfig index
> > > f532c59bb59b..877b43b3377d 100644
> > > --- a/drivers/misc/Kconfig
> > > +++ b/drivers/misc/Kconfig
> > > @@ -445,6 +445,17 @@ config HISI_HIKEY_USB
> > >  	  switching between the dual-role USB-C port and the USB-A host
> > ports
> > >  	  using only one USB controller.
> > >
> > > +config ZYNQ_AFI
> > > +	tristate "Xilinx ZYNQ AFI support"
> > > +	help
> > > +	  Zynq AFI driver support for writing to the AFI registers
> > > +	  for configuring PS_PL Bus-width. Xilinx Zynq SoC connect
> > > +	  the PS to the programmable logic (PL) through the AXI port.
> > > +	  This AXI port helps to establish the data path between the
> > > +	  PS and PL.In-order to establish the proper communication path
> > > +	  between PS and PL, the AXI port data path should be configured
> > > +	  with the proper Bus-width values
> > > +
> > >  source "drivers/misc/c2port/Kconfig"
> > >  source "drivers/misc/eeprom/Kconfig"
> > >  source "drivers/misc/cb710/Kconfig"
> > > diff --git a/drivers/misc/Makefile b/drivers/misc/Makefile index
> > > 99b6f15a3c70..e9b03843100f 100644
> > > --- a/drivers/misc/Makefile
> > > +++ b/drivers/misc/Makefile
> > > @@ -56,3 +56,4 @@ obj-$(CONFIG_HABANA_AI)		+=
> > habanalabs/
> > >  obj-$(CONFIG_UACCE)		+= uacce/
> > >  obj-$(CONFIG_XILINX_SDFEC)	+= xilinx_sdfec.o
> > >  obj-$(CONFIG_HISI_HIKEY_USB)	+= hisi_hikey_usb.o
> > > +obj-$(CONFIG_ZYNQ_AFI)		+= zynq-afi.o
> > > diff --git a/drivers/misc/zynq-afi.c b/drivers/misc/zynq-afi.c new
> > > file mode 100644 index 000000000000..04317d1bdb98
> > > --- /dev/null
> > > +++ b/drivers/misc/zynq-afi.c
> > > @@ -0,0 +1,81 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > > +/*
> > > + * Xilinx ZYNQ AFI driver.
> > > + * Copyright (c) 2018-2021 Xilinx Inc.
> > > + */
> > > +
> > > +#include <linux/err.h>
> > > +#include <linux/io.h>
> > > +#include <linux/module.h>
> > > +#include <linux/of.h>
> > > +#include <linux/platform_device.h>
> > > +
> > > +/* Registers and special values for doing register-based operations */
> > > +#define AFI_RDCHAN_CTRL_OFFSET	0x00
> > > +#define AFI_WRCHAN_CTRL_OFFSET	0x14
> > > +
> > > +#define AFI_BUSWIDTH_MASK	0x01
> > > +
> > > +/**
> > > + * struct afi_fpga - AFI register description
> > > + * @membase:	pointer to register struct
> > > + * @afi_width:	AFI bus width to be written
> > > + */
> > > +struct zynq_afi_fpga {
> > > +	void __iomem	*membase;
> > > +	u32		afi_width;
> > > +};
> > > +
> > > +static int zynq_afi_fpga_probe(struct platform_device *pdev) {
> > > +	struct zynq_afi_fpga *afi_fpga;
> > > +	struct resource *res;
> > > +	u32 reg_val;
> > > +	u32 val;
> > > +
> > > +	afi_fpga = devm_kzalloc(&pdev->dev, sizeof(*afi_fpga),
> > GFP_KERNEL);
> > > +	if (!afi_fpga)
> > > +		return -ENOMEM;
> > > +
> > > +	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
> > > +	afi_fpga->membase = devm_ioremap_resource(&pdev->dev, res);
> > > +	if (IS_ERR(afi_fpga->membase))
> > > +		return PTR_ERR(afi_fpga->membase);
> > > +
> > > +	val = device_property_read_u32(&pdev->dev, "xlnx,afi-width",
> > > +				       &afi_fpga->afi_width);
> > > +	if (val) {
> > > +		dev_err(&pdev->dev, "failed to get the afi bus width\n");
> > > +		return -EINVAL;
> > > +	}
> > > +
> > > +	reg_val = readl(afi_fpga->membase + AFI_RDCHAN_CTRL_OFFSET);
> > > +	reg_val &= ~AFI_BUSWIDTH_MASK;
> > > +	writel(reg_val | afi_fpga->afi_width,
> > > +	       afi_fpga->membase + AFI_RDCHAN_CTRL_OFFSET);
> > > +	reg_val = readl(afi_fpga->membase + AFI_WRCHAN_CTRL_OFFSET);
> > > +	reg_val &= ~AFI_BUSWIDTH_MASK;
> > > +	writel(reg_val | afi_fpga->afi_width,
> > > +	       afi_fpga->membase + AFI_WRCHAN_CTRL_OFFSET);
> > > +
> > > +	return 0;
> > > +}
> > 
> > I do not understand, why is this driver needed at all?  Why can't you do the
> > above from userspace?
> > 
> > All this does is write some values to the hardware at probe time, who needs
> > this?
> 
> This driver will be used by the overlay framework for configuring the interface after programming the FPGA and before probing the drivers that are present in the PL.

What is a "overlay framework"?  And why does the kernel have to do this?
Why can't you write these hardware values from userspace?

confused,

greg k-h