Re: [RFC v2 3/5] tpm_crb: Implement command and response chunking logic

[Arun Menon <armenon@redhat.com>]

On Sat, May 09, 2026 at 05:53:51PM +0300, Jarkko Sakkinen wrote:
> On Tue, Mar 24, 2026 at 11:42:42PM +0530, Arun Menon wrote:
> > With the introduction of support for Post Quantum Cryptography
> > algorithms in TPM, the commands and responses will grow in size.
> > Some TPMs have a physical hardware memory window (MMIO) that is
> > smaller than the commands we need to send. Therefore this commit
> > implements the core logic of sending/receiving data in chunks.
> > 
> > Instead of sending the whole command at once, the driver now sends it in
> > small chunks. After each chunk, it signals the TPM using a nextChunk
> > signal, and waits for the TPM to consume the data. Once the final piece
> > is delivered, the driver signals the TPM to begin execution by toggling
> > the start invoke bit. We use the same logic in reverse to read large
> > responses from the TPM.
> > 
> > This allows the driver to handle large payloads even when the hardware
> > interface has limited memory. This kernel-side support corresponds to
> > the backend implementation in QEMU [1]. QEMU reassembles the chunks
> > before passing them to the TPM emulator.
> > 
> > [1] https://lore.kernel.org/qemu-devel/20260319135316.37412-1-armenon@redhat.com/
> > 
> > Signed-off-by: Arun Menon <armenon@redhat.com>
> > ---
> 
> This is really bad at rationalizing the code change, which are heavy.
> 
> Couldn't you instead:
> 
> 1. Rename crb_recv as tpm_crb_recv_no_chunks().
> 2. Add tpm_crb_recv_chunks().
> 3. Add new tpm_crb_recv(), which delegates either.
> 
> I would not mind too much if they have some duplicate logic. It's less
> of a harm than convolution caused by interleaving.

Sure. I will amend this in v3.
Thank you.

> 
> >  drivers/char/tpm/tpm_crb.c | 155 +++++++++++++++++++++++++++----------
> >  1 file changed, 114 insertions(+), 41 deletions(-)
> > 
> > diff --git a/drivers/char/tpm/tpm_crb.c b/drivers/char/tpm/tpm_crb.c
> > index 922bcf7a69ad5..a97fc5e9927e3 100644
> > --- a/drivers/char/tpm/tpm_crb.c
> > +++ b/drivers/char/tpm/tpm_crb.c
> > @@ -104,11 +104,13 @@ struct crb_priv {
> >  	u8 __iomem *cmd;
> >  	u8 __iomem *rsp;
> >  	u32 cmd_size;
> > +	u32 rsp_size;
> >  	u32 smc_func_id;
> >  	u32 __iomem *pluton_start_addr;
> >  	u32 __iomem *pluton_reply_addr;
> >  	u8 ffa_flags;
> >  	u8 ffa_attributes;
> > +	bool chunking_supported;
> 
> Why not "u32 intf_id" i.e. cache the value of the register?
> 
> >  };
> >  
> >  struct tpm2_crb_smc {
> > @@ -368,38 +370,6 @@ static u8 crb_status(struct tpm_chip *chip)
> >  	return sts;
> >  }
> >  
> > -static int crb_recv(struct tpm_chip *chip, u8 *buf, size_t count)
> > -{
> > -	struct crb_priv *priv = dev_get_drvdata(&chip->dev);
> > -	unsigned int expected;
> > -
> > -	/* A sanity check that the upper layer wants to get at least the header
> > -	 * as that is the minimum size for any TPM response.
> > -	 */
> > -	if (count < TPM_HEADER_SIZE)
> > -		return -EIO;
> > -
> > -	/* If this bit is set, according to the spec, the TPM is in
> > -	 * unrecoverable condition.
> > -	 */
> > -	if (ioread32(&priv->regs_t->ctrl_sts) & CRB_CTRL_STS_ERROR)
> > -		return -EIO;
> > -
> > -	/* Read the first 8 bytes in order to get the length of the response.
> > -	 * We read exactly a quad word in order to make sure that the remaining
> > -	 * reads will be aligned.
> > -	 */
> > -	memcpy_fromio(buf, priv->rsp, 8);
> > -
> > -	expected = be32_to_cpup((__be32 *)&buf[2]);
> > -	if (expected > count || expected < TPM_HEADER_SIZE)
> > -		return -EIO;
> > -
> > -	memcpy_fromio(&buf[8], &priv->rsp[8], expected - 8);
> > -
> > -	return expected;
> > -}
> > -
> >  static int crb_do_acpi_start(struct tpm_chip *chip)
> >  {
> >  	union acpi_object *obj;
> > @@ -474,6 +444,8 @@ static int crb_trigger_tpm(struct tpm_chip *chip, u32 start_cmd)
> >  static int crb_send(struct tpm_chip *chip, u8 *buf, size_t bufsiz, size_t len)
> >  {
> >  	struct crb_priv *priv = dev_get_drvdata(&chip->dev);
> > +	size_t offset = 0;
> > +	size_t chunk_size;
> >  	int rc = 0;
> >  
> >  	/* Zero the cancel register so that the next command will not get
> > @@ -481,7 +453,7 @@ static int crb_send(struct tpm_chip *chip, u8 *buf, size_t bufsiz, size_t len)
> >  	 */
> >  	iowrite32(0, &priv->regs_t->ctrl_cancel);
> >  
> > -	if (len > priv->cmd_size) {
> > +	if (len > priv->cmd_size && !priv->chunking_supported) {
> >  		dev_err(&chip->dev, "invalid command count value %zd %d\n",
> >  			len, priv->cmd_size);
> >  		return -E2BIG;
> > @@ -491,18 +463,108 @@ static int crb_send(struct tpm_chip *chip, u8 *buf, size_t bufsiz, size_t len)
> >  	if (priv->sm == ACPI_TPM2_COMMAND_BUFFER_WITH_PLUTON)
> >  		__crb_cmd_ready(&chip->dev, priv, chip->locality);
> >  
> > -	memcpy_toio(priv->cmd, buf, len);
> > +	while (offset < len) {
> > +		chunk_size = min_t(size_t, len - offset, priv->cmd_size);
> >  
> > -	/* Make sure that cmd is populated before issuing start. */
> > -	wmb();
> > -
> > -	rc = crb_trigger_tpm(chip, CRB_START_INVOKE);
> > -	if (rc)
> > -		return rc;
> > +		if (chunk_size == 0)
> > +			break;
> >  
> > +		memcpy_toio(priv->cmd, buf + offset, chunk_size);
> > +		offset += chunk_size;
> > +
> > +		/* Make sure that cmd is populated before issuing start. */
> > +		wmb();
> > +		if (offset < len) {
> > +			rc = crb_trigger_tpm(chip, CRB_START_NEXT_CHUNK);
> > +			if (rc)
> > +				return rc;
> > +			if (!crb_wait_for_reg_32(&priv->regs_t->ctrl_start,
> > +			    CRB_START_NEXT_CHUNK, 0, TPM2_TIMEOUT_C)) {
> > +				dev_err(&chip->dev,
> > +					"Timeout waiting for backend to consume chunk\n");
> > +				return -ETIME;
> > +			}
> > +		} else {
> > +			rc = crb_trigger_tpm(chip, CRB_START_INVOKE);
> > +			if (rc)
> > +				return rc;
> > +		}
> > +	}
> >  	return crb_try_pluton_doorbell(priv, false);
> >  }
> >  
> > +static int crb_recv(struct tpm_chip *chip, u8 *buf, size_t count)
> > +{
> > +	struct crb_priv *priv = dev_get_drvdata(&chip->dev);
> > +	unsigned int expected;
> > +	size_t offset = 0;
> > +	size_t chunk_size;
> > +	size_t first_read;
> > +	int rc;
> > +
> > +	/* A sanity check that the upper layer wants to get at least the header
> > +	 * as that is the minimum size for any TPM response.
> > +	 */
> > +	if (count < TPM_HEADER_SIZE)
> > +		return -EIO;
> > +
> > +	/* If this bit is set, according to the spec, the TPM is in
> > +	 * unrecoverable condition.
> > +	 */
> > +	if (ioread32(&priv->regs_t->ctrl_sts) & CRB_CTRL_STS_ERROR)
> > +		return -EIO;
> > +
> > +	/* Read the first 8 bytes in order to get the length of the response.
> > +	 * We read exactly a quad word in order to make sure that the remaining
> > +	 * reads will be aligned.
> > +	 */
> > +	memcpy_fromio(buf, priv->rsp, 8);
> > +
> > +	expected = be32_to_cpup((__be32 *)&buf[2]);
> > +	if (expected > count || expected < TPM_HEADER_SIZE)
> > +		return -EIO;
> > +
> > +	/*
> > +	 * Set chunk_size by comparing the size of the buffer that the upper layer has
> > +	 * allocated (count) to the hardware tpm limit (priv->rsp_size).
> > +	 * This is to prevent buffer overflow while writing to buf.
> > +	 */
> > +	chunk_size = min_t(size_t, count, priv->rsp_size);
> > +	if (chunk_size < 8)
> > +		return -EIO;
> > +
> > +	/*
> > +	 * Compare the actual size of the response we found in the header to the chunk_size.
> > +	 */
> > +	first_read = min_t(size_t, expected, chunk_size);
> > +
> > +	memcpy_fromio(&buf[8], &priv->rsp[8], first_read - 8);
> > +	offset = first_read;
> > +
> > +	while (offset < expected) {
> > +		if (!priv->chunking_supported) {
> > +			dev_err(&chip->dev, "Response larger than MMIO and chunking not supported\n");
> > +			return -EIO;
> > +		}
> > +
> > +		rc = crb_trigger_tpm(chip, CRB_START_NEXT_CHUNK);
> > +		if (rc)
> > +			return rc;
> > +
> > +		if (!crb_wait_for_reg_32(&priv->regs_t->ctrl_start,
> > +					 CRB_START_NEXT_CHUNK, 0, TPM2_TIMEOUT_C)) {
> > +			dev_err(&chip->dev, "Timeout waiting for backend response\n");
> > +			return -ETIME;
> > +		}
> > +
> > +		chunk_size = min_t(size_t, expected - offset, priv->rsp_size);
> > +		memcpy_fromio(buf + offset, priv->rsp, chunk_size);
> > +		offset += chunk_size;
> > +	}
> > +
> > +	return expected;
> > +}
> > +
> >  static void crb_cancel(struct tpm_chip *chip)
> >  {
> >  	struct crb_priv *priv = dev_get_drvdata(&chip->dev);
> > @@ -727,6 +789,15 @@ static int crb_map_io(struct acpi_device *device, struct crb_priv *priv,
> >  		goto out;
> >  	}
> >  
> > +	if (priv->regs_h) {
> > +		u32 intf_id = ioread32((u32 __iomem *)&priv->regs_h->intf_id);
> > +
> > +		if (intf_id & CRB_INTF_CAP_CRB_CHUNK) {
> > +			priv->chunking_supported = true;
> > +			dev_info(dev, "CRB Chunking is supported by backend\n");
> > +		}
> > +	}
> > +
> >  	memcpy_fromio(&__rsp_pa, &priv->regs_t->ctrl_rsp_pa, 8);
> >  	rsp_pa = le64_to_cpu(__rsp_pa);
> >  	rsp_size = ioread32(&priv->regs_t->ctrl_rsp_size);
> > @@ -764,8 +835,10 @@ static int crb_map_io(struct acpi_device *device, struct crb_priv *priv,
> >  	priv->rsp = priv->cmd;
> >  
> >  out:
> > -	if (!ret)
> > +	if (!ret) {
> >  		priv->cmd_size = cmd_size;
> > +		priv->rsp_size = rsp_size;
> > +	}
> >  
> >  	__crb_go_idle(dev, priv, 0);
> >  
> > -- 
> > 2.53.0
> > 
> 
> BR, Jarkko
> 

Regards,
Arun Menon