Re: [PATCH v4 4/4] tty: serial: 8250: add DFL bus driver for Altera 16550.

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On Tue, 1 Nov 2022, Ilpo Järvinen wrote:

On Tue, 1 Nov 2022, matthew.gerlach@xxxxxxxxxxxxxxx wrote:

On Tue, 1 Nov 2022, Xu Yilun wrote:

On 2022-10-31 at 17:34:39 -0700, matthew.gerlach@xxxxxxxxxxxxxxx wrote:

On Sat, 29 Oct 2022, Xu Yilun wrote:

On 2022-10-20 at 14:26:10 -0700, matthew.gerlach@xxxxxxxxxxxxxxx wrote:
From: Matthew Gerlach <matthew.gerlach@xxxxxxxxxxxxxxx>

Add a Device Feature List (DFL) bus driver for the Altera
16550 implementation of UART.

Signed-off-by: Matthew Gerlach <matthew.gerlach@xxxxxxxxxxxxxxx>
v4: use dev_err_probe() everywhere that is appropriate
    clean up noise
    change error messages to use the word, unsupported
    tried again to sort Makefile and KConfig better
    reorder probe function for easier error handling
    use new dfh_find_param API

v3: use passed in location of registers
    use cleaned up functions for parsing parameters

v2: clean up error messages
    alphabetize header files
    fix 'missing prototype' error by making function static
    tried to sort Makefile and Kconfig better
 drivers/tty/serial/8250/8250_dfl.c | 149
 drivers/tty/serial/8250/Kconfig    |  12 +++
 drivers/tty/serial/8250/Makefile   |   1 +
 3 files changed, 162 insertions(+)
 create mode 100644 drivers/tty/serial/8250/8250_dfl.c

diff --git a/drivers/tty/serial/8250/8250_dfl.c
new file mode 100644
index 000000000000..f02f0ba2a565
--- /dev/null
+++ b/drivers/tty/serial/8250/8250_dfl.c
@@ -0,0 +1,149 @@
+// SPDX-License-Identifier: GPL-2.0
+ * Driver for FPGA UART
+ *
+ * Copyright (C) 2022 Intel Corporation, Inc.
+ *
+ * Authors:
+ *   Ananda Ravuri <ananda.ravuri@xxxxxxxxx>
+ *   Matthew Gerlach <matthew.gerlach@xxxxxxxxxxxxxxx>
+ */
+#include <linux/bitfield.h>
+#include <linux/dfl.h>
+#include <linux/io-64-nonatomic-lo-hi.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/serial.h>
+#include <linux/serial_8250.h>
+struct dfl_uart {
+	int line;
+static int dfl_uart_get_params(struct dfl_device *dfl_dev, struct
uart_8250_port *uart)
+	struct device *dev = &dfl_dev->dev;
+	u64 v, fifo_len, reg_width;
+	u64 *p;
+	p = dfh_find_param(dfl_dev, DFHv1_PARAM_ID_CLK_FRQ);
+	if (!p)
+		return dev_err_probe(dev, -EINVAL, "missing CLK_FRQ
+	uart->port.uartclk = *p;
+	dev_dbg(dev, "UART_CLK_ID %u Hz\n", uart->port.uartclk);
+	p = dfh_find_param(dfl_dev, DFHv1_PARAM_ID_FIFO_LEN);
+	if (!p)
+		return dev_err_probe(dev, -EINVAL, "missing FIFO_LEN
+	fifo_len = *p;
+	dev_dbg(dev, "UART_FIFO_ID fifo_len %llu\n", fifo_len);
+	switch (fifo_len) {
+	case 32:
+		uart->port.type = PORT_ALTR_16550_F32;
+		break;
+	case 64:
+		uart->port.type = PORT_ALTR_16550_F64;
+		break;
+	case 128:
+		uart->port.type = PORT_ALTR_16550_F128;
+		break;
+	default:
+		return dev_err_probe(dev, -EINVAL, "unsupported
fifo_len %llu\n", fifo_len);
+	}
+	p = dfh_find_param(dfl_dev, DFHv1_PARAM_ID_REG_LAYOUT);
+	if (!p)
+		return dev_err_probe(dev, -EINVAL, "missing REG_LAYOUT
+	v = *p;
+	uart->port.regshift = FIELD_GET(DFHv1_PARAM_ID_REG_SHIFT, v);
+	reg_width = FIELD_GET(DFHv1_PARAM_ID_REG_WIDTH, v);

I have concern that the raw layout inside the parameter block is
still exposed to drivers and need to be parsed by each driver.

Raw parameter block will always have to be passed to the driver because HW
specific properties can be defined that will need to be parsed by the
specific driver.

So there is a question about the scope of the definitions of these parameter
blocks. MSIX seems globally used across all dfl devices. REG_LAYOUT
seems specific to uart?

There are definitely two classes of parameter blocks.  One class is HW
agnostic parameters where the parameters are relevant to many different kinds
of HW components.  MSI-X, and input clock-frequency are certainly HW agnostic,
and it turns out that REG_LAYOUT is not specific to uart.  You can see
reg_bits and reg_stride in struct regmap_config.  There are also device tree
bindings for reg-shift and reg-io-width.  The second class of parameters would
be specific to HW component.  In the case of this uart driver, all parameters
would be considered HW agnostic parameters.

If a parameter block is widely used in dfl drivers, duplicate the parsing
from HW layout in each driver may not be a good idea. While for device
specific parameter block, it's OK.

It sounds like we are in agreement.

Another concern is the indexing of the parameter IDs. If some parameter
blocks should be device specific, then no need to have globally indexed
parameter IDs. Index them locally in device is OK. So put the definitions
of ID values, HW layout and their parsing operation in each driver.

It may be confusing for two drivers to use the same parameter id that have
different meanings and data layout.  Since all the parameters for this driver
would be considered HW agnostic, we'd don't need to address this issue with
this patchset.

How about we define HW agnostic IDs for parameter specific fields like:


And define like u64 dfl_find_param(struct dfl_device *, int param_id,
int field_id)

I don't think dfl_find_param as defined above adds much value.

Think further, if we have to define HW agnostic property - value pairs,
why don't we just use "Software nodes for the firmware node", see
drivers/base/swnode.c. I think this may be a better choice.

I am looking into "Software nodes for the firmware node", and it can be
for HW agnostic properties.  Each dfl driver will still have to make a
function call to fetch each HW agnostice property value as well as a
function call to find the HW specific parameters and then parse those

Btw, another aspect this discussion has completely overlooked is the
presence of parameter version and how it impacts data layout. Is v1
always going be a subset of v2 or can a later version remove something
v1 had?

In general it would be preferable for v1 to be a subset of v2. This allows for v1 SW to work on v2 HW.


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